Acetic, propionic and butyric acids are the major fermentation acids produced on acidogenesis of organic wastes such as food scraps. They can be utilized by Ralstonia eutropha as sole carbon sources for cell growth and polyhydroxyalkanoate (PHA) synthesis. The acids, however, are inhibitory and toxic to the bacterium depending on the medium pH and the total acid concentration. The inhibition and utilization kinetics of the total acids and individual acids in mixed acid media were investigated in flask batch cultures, and simulated by modified Michaelis-Menten models based on variable cell activity. Cell activity, a function of the total acid concentration, is defined as the active fraction of the measurable residual biomass (RBM) under specific culture conditions, which might be as low as 20% at high acid concentrations. The overall production rate of PHA is contributed from the utilization rates of individual acids as well as the interaction of two acids such as acetate and propionate, depending on the predominant acids in the medium. R. eutropha preferred propionic acid for cell mass synthesis and butyric acid for polymer synthesis, the latter giving the highest polymer yield (0.39 g/g) among the three acids. In mixtures of predominant acetic and butyric acids, a high PHA formation rate (60 mg/g RBM per h) was achieved, compared with the relatively low PHA formation rate (35 mg/g RBM per h) in the mixtures of predominant acetic and propionic acids. Acetic acid in acid mixtures reduced the metabolism of propionic acid, which resulted in a high rate ratio (up to 0.8) of hydroxyvalerate (HV) formation to the overall PHA formation. (C) 2002 Elsevier Science Ltd. All rights reserved.