Anaerobic digestion (AD) is the most often applied technique for sewage sludge stabilization at medium and large wastewater treatment plants. Nonetheless, the application of AD to sewage sludge stabilization is often limited by long retention time and low VS (volatile solid) removal. For this reason in recent years a number of studies have been focused on optimize anaerobic digestion processes. One of the most interesting options for improving anaerobic digestion yields is co-digestion, namely simultaneous decomposition of a homogenous mixture of at least two biodegradable wastes. Fat-rich materials are attractive substrate for AD due to the high organic matter content of waste and high energetic potential. It is estimated that about 1,014 dm3, methane at STP (standard temperature and pressure) can by produced from 1 g VS lipids, while only 0,415, 0,496 dm3 can be produced respectively from 1 g VS carbohydrate and protein. However, due to inhibitory eﬀect of intermediate compounds (LCFAs – long chain fatty acids) and operational problems, such as: hindrance, sedimentation clogging, scum formation, and flotation of biomass, separate AD of this waste ended with failure. The aim of the current study was to investigate how the co-digestion of fats of vegetable origin (FV) and mixed sewage sludge affected the performance of the anaerobic digestion (AD) process. The process was carried out at mesophilic conditions (37°C) in continuous stirred-tank reactor (CSTR) with working liquid volume equal to 6,5 dm3. The reactors were operated in draw-and fill mode (on a daily basis). The digestion was examined in semi-continuous mode at sludge retention time of 10 days and the organic loading rate maintained in the range 2,24–3,02 g/dm3 d. During the start-up period the digester was fed only sewage sludge. Co-digestion process was initiated after achievement of stable working parameters of bioreactor for sewage sludge digestion. Addition of fat in the feedstock was gradually increased up to 35%. Anaerobic process state indicators such as: biogas production, biogas composition, pH, alkalinity and volatile fatty acids (VFA) were used to monitor a digestion. Furthermore, the LCFAs concentrations were measured in a feed and digested sludge. The results showed that use of FV as a co-substrate adversely affects the efficiency of the process. Comparing digestion of MSS alone with co-digestion of wastes, it was shown that co-digestion resulted in lower biogas production and VS removal. Biogas yields for co-digestion mixtures were between 0,16 and0,32 dm3/g VS added, while volatile solid (VS) removal ranged from 36,75 to 42,65%.However average biogas yield and VS degradation degree observed during fermentation of the MSS(mixed sewage sludge) alone were 0,33 dm3/g VS added and 44%, respectively. Only for biogas composition noted the positive effect of mixed sewage sludge co-fermentation with FV. The study showed that the concentration of ammonia generated in this experiment did not inhibit anaerobic digestion. It was found that oleic acid, which is one of the most toxic long chain fatty acids, was present at concentrations (reached a maximum value of 34,38 mg/g TS) within the ranges for which inhibition of methanogenesis has been reported. This suggests that the low efficiency of the process was probably caused a high concentration of oleic acid.