Factorial designs were employed to analyze the inhibitory effects of acetic, butyric, lactic, and propionic acids on composting microorganisms. Compost samples were withdrawn on different days of composting and treated with acids alone and in combination (at 0 and 0.5 mmol/g). Microorganisms were enumerated to determine degree of growth inhibition. Generally, inhibition was more severe on the day when pH decreased rather than on the day when pH started to increase. Butyric or lactic acid alone, and the combination of butyric, lactic, and propionic acids, significantly inhibited thermophilic bacteria. Only 51.0-65.0% of the thermophilic bacteria exist if 0.5 mmol/g of these acids were present in compost. Temperature, microbial populations, and microbial growth phase might cause variation in the inhibitory effects of acids. These findings are useful not only in the study of microorganisms in acidic microenvironments, but also in preventing microbial-growth inhibition by predicting population via a regression model. (C) 2010 Elsevier Ltd. All rights reserved.