Bean curd refuse-okara, a byproduct of tofu (soybean cake) production, contains more than 20% total solid (TS), making it difficult to use as a substrate for anaerobic methane fermentation without dilution. This study focused on the batch methane fermentation of okara in anaerobic. digesters under mesophilic (36 degrees C) conditions. Inoculum was mixed with different amounts of okara, and their final moisture content was adjusted to 90%. Methane production at 13 different substrate-to-inoculum ratios (S/Is), ranging from 0.1 to 3.0, were determined. The results indicated that despite the TS loading of 10%, which is significantly higher than that of traditional methane fermentation, okara was fermented to methane at the highest methane-based degradation of 93%. When the S/I values ranged from 0.1 to 0.6, methane yields indicated a linear relationship with the S/I value. Methane yield reached its maximum at S/I values of 0.6-0.9. In contrast, methane yield decreased when the S/I exceeded 1.0, due to higher accumulation of volatile fatty acid (VFA), which significantly inhibits fermentation. After 19 days of mesophilic digestion, maximum methane yields of 478-495 ml CH4 g VS-1 were achieved at S/I values of 0.6-0.9, which corresponds to 90-93% methane-based degradation of okara. (C) 2011 Elsevier Ltd. All rights reserved.