This paper presents the results of an experimental batch methane fermentation (at 37 degrees C mesophilic temperature) study carried out on untreated and pretreated substrates of rice straw using NaOH and hydrothermal pretreatments. 3% NaOH pretreatment was given to ground rice straw biomass for 120 h at 37 degrees C and hydrothermal pretreatment was given for 10 min at 200 degrees C. It was observed that NaOH addition is a mandatory requirement for maintaining a suitable range of pH and starting the biogas production from hydrothermal pretreated biomass slurry of rice straw. The fed substrate concentrations were maintained at 5% TS (50 g TS/L). The study revealed into 140.0 L/kg VSa biogas and 59.8 L/kg VSa methane from untreated rice straw substrate. However, NaOH pretreated substrate resulted into 184.8 L/kg VSa biogas and 74.1 L/kg VSa methane. Hydrothermal pretreated followed by 5% NaOH added substrate resulted into highest biogas and methane production yields as 315.9 L/kg VSa and 132.7 L/kg VSa, respectively. NaOH pretreated substrate showed an increase of 132.0% in biogas production and 123.9% in methane production relative to the untreated substrate. However, the hydrothermal pretreated substrate had resulted into an increase of 225.6% in biogas production and 222.0% in methane production relative to untreated rice straw substrate. Hydrothermal pretreatment provided an accelerated pre-hydrolysis of biomass contents during the treatment process and thereby resulted into enhanced biogas and methane production yields. (c) 2012 Elsevier Ltd. All rights reserved.