Unsustainable depletion of resources and large amounts of waste have an adverse impact on the environment. After using a walnut kernel, large amounts of shell remain, which represent unused residue. Recently, the possibilities of using biomass to produce environmentally friendly biocomposites have increased significantly, and the walnut shell has the quality properties of biomass to produce biocomposites. Due to the lower amounts of hygroscopic components and higher amounts of hydrophobic components in shell walnut biomass compared to forest biomass, polymer-based composite materials containing walnut shell fillers have significant commercial advantages in outdoor products that require high resistance. Also, walnut shell represents a very useful biomass that is used in many production processes. Hydrolysis pretreatment includes the process of processing lignocellulosic raw material at pressure and temperature, during which hemicellulose and cellulose are hydrolyzed into simple carbohydrates that can later be used in biorefineries to produce fuel, but also other products such as biocomposites and liquefied biomass. Hydrolysis is a process of chemical treatment of lignocellulosic biomass, and the goal is to degrade lignin from the material. In the process of hydrolysis, chemical bonds break due to the insertion of water molecules between the atoms in the bonds, that is, large molecules in an aggressive medium are hydrolyzed into smaller ones. The research consisted of several phases: collection of samples, basic physical analyses, mechanical pre-treatment, basic chemical analyses, pre-treatment of walnut shells by hydrolysis in three ways: with acid, alkali and alcohol under the following conditions: time (15 min, 30 min, 45 min), temperature (80°C, 90°C, 100°C, 110°C, 120°C), analysis of the chemical structure of walnut shell after pretreatment, liquefaction of walnut shell biomass and the analysis of characteristics of liquefied biomass. Walnut shells are generally characterized by a high content of lignin, and the high lignin content in walnut shells enables a special firm and dense structure of the cell wall. The pretreatment achieved a reduction in the proportion of lignin in all investigated samples comparing to untreated sample (53.87%). Comparing the highest and lowest values of the proportion of lignin depending on the type of hydrolysis, it was determined that the lowest proportion (42.33%) was achieved by alkaline hydrolysis, followed by acid hydrolysis (42.72%), while the lowest proportion of lignin after alcohol hydrolysis was 45.66%. The pretreatment also achieved an increase in the cellulose content of all investigated samples, regardless of the type of hydrolysis, temperature, and duration. After alkaline hydrolysis, the highest proportions of cellulose were determined (42.26% at 120 °C for 45 minutes). At the same time, the pretreatment with hydrolysis did not significantly affect the proportion of hemicellulose. The pretreatment significantly reduced the proportion of extractives in almost all samples (except those that underwent acid hydrolysis at 100 °C for 15 minutes). It can be determined that after the alcohol hydrolysis, the lowest proportions of extractive substances were determined (0.31% at 120 °C for 30 minutes). Pretreatment also reduced the ash content in all samples after acid and alcohol hydrolysis, while in the samples after alkaline hydrolysis the ash content was significantly higher compared to the untreated sample before hydrolysis. Acid hydrolysis had the most significant effect on reducing the ash content since the highest ash content was only 0.16% (at 110 °C for 30 and 45 minutes). Samples after hydrolysis with alcohol had the lowest degree of liquefaction (85.00% at 120 °C for 45 min), while the highest degree of liquefaction was found in samples after alkaline hydrolysis (90.76% at 90 °C for 15 minutes). According to the functional groups of natural walnut shell biomass components formed during the addition of polyhydric alcohols, glycols and organic acids that are activated by the liquefaction process, the liquefied walnut shell biomass has further applications in various bioproducts and represents a useful source for further synthesis into various bioproducts. The high amount of lignin makes the walnut shell a suitable raw material for the production of glue, which can provide added value to the agricultural sector, i.e. walnut production. Likewise, walnut shell biomass, due to its group chemical composition, is suitable for the preparation of resinous materials such as polyurethane foam and polyurethane glue, epoxy resins and urea-urethane elastomers. Also, the pretreatment achieved a significant increase in the OH-number regardless of the type of hydrolysis, temperature and duration of the pretreatment compared to the untreated sample (385.67 KOH/g). The samples after alkali hydrolysis had the highest OH-number (1288.03 KOH/g) compared to all mean OH-number values after pretreatment with acid and alcohol. Based on all of the above, the research confirms the first and third hypotheses - (H1) walnut shell has quality properties of biomass for biocomposite production and (H3) - higher temperature and duration of pretreatment depending on the type of reagent will have a positive effect on the quality and quantity of biocomposite, while the second hypothesis – (H2) the process of alcoholic hydrolysis of the walnut shell will yield biocomposites with a more favorable composition of lignin, cellulose and hemicellulose and better polymer properties – is rejected.