This study was conducted to advance the understanding of thermophilic grass digestion. Late harvested grass silage was fermented at thermophilic conditions at increasing organic loading rates ( OLR). Stable digestion took place at an OLR between 3 and 4 g VS L (1) d (1). This enabled specific methane yields ( SMY) as high as 405 L CH4 kg VS (1). An accumulation of volatile fatty acids ( VFA), accompanied by a gradual deterioration of pH, FOS/TAC ( ratio of VFA to alkalinity) arose at an OLR between 5 and 7 g VS L (1) d (1), yet inhibition did not occur. SMY decreased with reduced retention time ranging between 336 and 358 L CH4 kg VS (1) at OLR 7 and 5 g VS L (1) d (1) respectively. The biomethane efficiencies remained high ( 92-103%) at corresponding retention times. Comparative results indicated a superior performance with respect to higher loading and SMY as compared with mesophilic conditions. (C) 2016 Elsevier Ltd. All rights reserved.