Acidogenesis affect on not only methanogenic but also acidogenic and hydrolysis phases in the overall anaerobic process of solid-state organic waste. A batch experiment was conducted to evaluate the influence of unionized volatile acid (UVA) on acidogenesis and hydrolysis of high-solids organic waste. The temperature regime was mesophilic with a 20 day solid retention time. The reactor set-up was a rotational drum fermentation system (RDFS) consisting of 4 fermentors and using soybean meal as a substrate. Results showed that UVA inhibited strongly on the VA increasing ratios, spectra, VS degradation ratios, and the surface based hydrolysis constant with increasing initially added UVA concentration (0, 10, 20, 30 g/L). Results suggested that VA, particularly UVA, strongly inhibited hydrolysis and acidogenesis during anaerobic solid-state substrate degradation.