Single and combined inhibition of lag time lambda and specific methanogenic activity R-CH4 of Methanosaeta concilii by NH3, Na+ and H2S were investigated using inhibition tests with a single inhibitor and a 3(3) full-factorial experiment of NH3, Na+ and H2S concentrations (1.5 <= total ammonia nitrogen (TAN)/L <= 4.5 g, 1 <= Na+/L <= 4.3 g, 14.2 <= total hydrogen sulfide sulfur (THSS)/L <= 836 mg). All three inhibitors significantly increased. and reduced R-CH4 of M. concilii. The half-maximal inhibitory concentrations of NH3, Na+ and H2S for M. concilii were 6.4 g TAN/L, 5.2 g Na+/L and 1.6 g THSS/L. Partial cubic models adequately approximated the corresponding response surfaces of lambda and R-CH4 from the 3(3) full-factorial experiment. The inhibitors inhibited R-CH4 synergistically, but inhibited lambda in a complex manner. The combination of NH3 and Na+ showed the strongest synergistic inhibition of both lambda and R-CH4.