Two nitrification‐denitrification systems were used to determine the possible inhibitory and/or toxic effects of supplemental chromium and nickel addition to the nitrification‐denitrification process in the treatment of a relatively high ammonia, low biodegradable carbon, municipal landfill leachate. Results indicated that chromium (Cr+3) and nickel (Ni+2) caused inhibitory effects to the system at soluble concentrations of approximately 0.30 and 0.70 mg/L, respectively. Chromium affected both the nitrification and denitrification process, whereas nickel impaired only the nitrification performance; denitrification was unaffected at the nickel concentrations used. The failures were believed to be due to either the soluble metal levels themselves or the buildup of metals in the solids, although lack of long‐term acclimation cannot be ruled out. No phosphorus deficiency was noted in either case. This ruled out the possibility of system failures due to lack of bioavailable phosphorus, as reported with zinc in the early stages of this study.