Toxicity of zinc, cadmium and phenol and their binary mixtures were determined based on inhibition of dehydrogenase activities of yeasts strains isolated from hydrocarbon impacted soil. The yeasts isolated from the hydrocarbon impacted soil were identified as Saprochaete sp. and Cryptococcus sp. The toxicity of chemicals and their mixtures were evaluated in the concentration range of 0 - 16 mM for phenol, 0 - 5 mM for zinc and 0 - 0.4 mM for cadmium. The binary mixture ratios were evaluated. The toxicity thresholds (IC50) were estimated using 3-parameter logistic dose-response model. The IC50 shows that cadmium has the highest toxic effect on the yeasts with IC50 value of 0.075 mM for Saprochaete sp. and 0.09 mM for Cryptococcus sp. The binary toxicity of the mixtures on the enzyme activity of the yeasts was evaluated with toxic index (TI) model. Modulation of the toxic interactions by the components of the mixtures through synergistic, additive and antagonistic interactions on the heavy metals and phenol were possible against the dehydrogenase activity of the yeasts. However, the toxic effects of phenol and heavy metals would depend on their relative amount present in the environment.