The goal of this research was to appraise the effect of combined inoculation on the performance of anaerobic digesters treating hardly degradable material, and particularly the pressed fine sieved fraction (PFSF) derived from wastewater treatment plants (WWTPs). Batch tests were conducted in mesophilic conditions in order to examine the optimal mixing ratio of inoculums. Mixing ratios of 100:0, 75:25, 50:50, 25:75, and 0:100 of three different inoculums were applied in the batch tests. The findings indicated that the inoculation of digested activated sludge with digested organic fraction of municipal solid waste (MSW) in the ratio 25:75 resulted in a higher PFSF degradation and a higher biogas yield. The results from the kinetic analysis fit well with the results from the batch experiment.