Abstract Phospholipid fatty acid (PLFA) analysis was used to characterize microbial community structure during the composting of grape pomace and rice straw. Composting studies were completed in 30 litre static-bed reactors with continuous temperature and oxygen monitoring. The effects of inoculation and pH adjustment on microbial community structure and level of decomposition during composting were investigated. Principal components analysis (PCA) of the PLFA data showed that inoculation had little effect on the microbial community structure of the compost once temperature had peaked, while process temperature and the adjustment of initial pH had a significant effect. Adjustment of pH and inoculation did not significantly increase the level of decomposition as measured by oxygen consumption.