A low level of microbial activity due to the production of organic acids is a recognized problem during the initial phase of food waste composting. Increasing such activity levels by adjusting the pH values during the initial composting phase was the primary concern to be investigated. In this study, sodium acetate (NaAc) was introduced as an amendment to an in-vessel composting system. NaAc was added when the pH of the compost mixture reached a low level (pH < 5), and its effects on microbial activity, ammonia loss, and organic acid production were then evaluated. The addition of NaAc would lead to an increased pH level within the range from 5.2 to 5.5. This had a positive effect on the degradation of organic materials and the effect was statistically significant compared to the result of control treatment without NaAc addition (p < 0.05). Microbial activity in the composting reactor treated with NaAc was also higher than that of the control one after the indigenous microorganisms adapted to the new conditions. However, the microbial populations of these two reactors were not significantly different. Although, ammonia loss was enhanced with the addition of NaAc, with 10.8 and 8.6 gin NaAc amendment reactor and control one, respectively, the degree of enhancement was relatively small compared to the total amount of nitrogen in the raw materials (84 g). The study results indicated that the NaAc was an effective amendment for inhibiting the production of propionic and butyric acids, and hence counteracting the adverse effects of organic acids to the composting process. Crown Copyright (c) 2008 Published by Elsevier Ltd. All rights reserved.