The aim of this study was to investigate anaerobic digestion of wheatgrass in the absence of hydrolytic pre-treatments. The effect of solids retention time (SRT) (1-64 days), inoculum acclimation (0-80 days acclimation), temperature (40-70°C) and buffer capacity (20-200 mM phosphate buffer) on conversion of the feedstock, yield and composition of liquid-phase products (ethanol and short-chain organic acids, SCOAs) and COD removal was investigated in semi-continuous (intermittent feed) completely mixed reactors. SRT had the most important effect on process performance. Biodegradation of the feedstock was favoured at high SRT, with 61% removal of volatile suspended solids and 84% removal of total carbohydrates at SRT 64 days. However, low yield of liquid-phase products was observed at high SRT because of strong methanogenic activity (57% removal of the total COD). The highest yield of liquid-phase products was 20% (COD basis) at SRT 8 days. Although high biodegradation of the feedstock was observed after long-term batch acclimation (30 and 80 days), once the digestion conditions were switched to semicontinuous at short SRT (2 days), the biodegradation of the feedstock decreased considerably. The best process performance was observed at 40°C.