The use of microorganisms for the biodegradation of pollutants is increasingly being studied. But at high concentrations, these pollutants become rather inhibitors for the metabolism of microorganisms. In this study, the biodegradation of ammonium formate at various concentrations (1.59-7.94 mM) by Yarrowia Lipolytica and Pichia guilliermondii isolated from the rubber effluent is studied by following the variation of ammonium ions and formate. A fitting of eight models of substrate inhibition was performed and the parameters were determined by nonlinear regression using MATLAB 8.5 A (c). The R (2) and the RSME allow to choose the best model. The results show that ammonium ions (3.17 mM ammonium formate) are used as substrate; no inhibition is observed. But beyond this concentration, the inhibition effect begins to be observed with the specific rates of ammonium biodegradation which decrease. Formate monitoring reveals that is used as the main source of energy and does not inhibit the growth of yeasts. The models of Luong and Webb seem to be more appropriate for predicting the observed phenomena of inhibition. For Yarrowia lipolytica, R (2) = 0.958 and 0.998 with RSME = 0.005342 and 0.003433, for Pichia guillermondii, R (2) = 0.999 and 0.992 with RSME = 0.0005121 and 0.001212.