In this paper, two experiments (P1 and P2) of composting were carried out in two reactors (R1 and R2) with different characteristics and working volume of Vr = 10 L. The aim of these experiments was to determine whether the reactor characteristics affect the composting process as well as the composition of biowaste. Experiment P1 was conducted for 16 days, and the composition of biowaste was not defined, while in experiment P2, the composition of biowaste was defined and experiment was conducted for 14 days. In certain periods, depending on the temperature, a sample of the substrate was taken and physicochemical indicators (pH-value of the sample, concentration of dissolved oxygen, electrical conductivity, moisture content, volatile and dry substances in the sample) and microbiological indicators (total number of living bacteria and fungi) were determined. At the end of the experiment, an analysis of total organic carbon, TOC and ecotoxicity test using the marine bacterium Vibrio fischeri were performed on substrate samples and leachate samples. The maximum temperature achieved in P1 was 33,4 ℃ in R1 and 36,1 ℃ in R2, while in P2 the maximum temperature achieved was 43,3 ℃ in R1 and 44,7 ℃ in R2. During experiment P1, the pH value of the substrate increased from 5,13 to 7,79 in R1 and 7,84 in R2, and during experiment P2 from 6,11 to 8,58 in R1 and 8,45 in R2. The results of the ecotoxicity test showed that the substrate eluates and leachate were toxic in both experiments. A conversion of 24,99 % in R1 and 23,76 % in R2 of the experiment P1 and 37,19 % in R1 and 56,92 % in R2 of the experiment P2 was achieved.