The main purpose of this work is the characterization of electrical properties of two types of coke (metallurgical and petrochemical) of different origin and their mixtures for comparison of the coke coating filler efficiency for inert MMO anodes in the cathode protection system with external current source. Metallurgical coke (MET) is the final product obtained by dry distillation of coal in anaerobic conditions, that is without the access of air, and petrochemical coke (PET) is obtained as a secondary product of the rafinery process of heat coking of heavy petrolium residues. Prepared test samples of 100 % MET and 100 % PET coke and their mixtures with different proportions, 70 % MET - 30 % PET and 50 % MET - 50 % PET coke encased four MMO anodes of same dimensions and are buried in separate anode reservoirs. Anodes with an external source of current are connected from the individual anode deposits, that is with four connected power stations which mantained constant direct current to the anode reservoirs, causing the consequent degradations of coke fillers around the MMO anodes. The test results showed that the best protection efficiency for an inert anode in an anodised bed provides a coke filler with 100 % PET coke since it doubles the grounding resistance of an anode MMO over time and prolongs the longevity of the anode, but at a 3.7x higher price than coke fillers with 100 % MET coke content, which doubled the grounding resistance of the MMO anode in the anode reservoir with the time of measurement and showed the lowest absorption of soil water. In contrast, the high water absorption from the ground, i.e., better resistance to degradation due to the constant transient direct current, showed 100 % PET coke filler following the time oscillation in the measured electrical resistance of the soil, thus providing a primary surface for conducting the anode oxidation reaction C to CO or CO2. Mixed fills of two types of coke: 50 % MET - 50 % PET and 70 % MET - 30 % PET coke showed the electrical properties of both types of coke, but improved efficacy of the MMO anode protection in the anode reservoir has nevertheless shown a mixture with a higher proportion of PET coke.