The effect of two additives on anaerobic monodigestion of pig slaughterhouse waste was studied in three laboratory scale continuously stirred tank reactors (CSTRs) operated for 242–284days at 35°C. A higher organic loading rate (OLR), namely 2.25kg volatile solids (VS)/m 3 d with a hydraulic retention time (HRT) of 30d was feasible with an additive containing Fe, HCl and trace elements while an OLR of 1.5kg VS/m 3 d (HRT 30d) led to instabilities in CSTR with an additive consisting of Fe and HCl and in CSTR without additives. Methane yields of ca. 700dm 3 CH 4 /kg VS fed were obtained in all reactors under stable conditions. Both additives decreased H 2 S concentration of the biogas below detection limit 0.5ppm while in a control reactor H 2 S concentration was up to 1900ppm. In conclusion, the results suggest the possibility that additives may facilitate higher OLRs in monodigestion of pig slaughterhouse waste.