Rice straw was pretreated with sodium hydroxide and subsequently conditioned to reduce the pH to 5-6 by either: (1) extensive water washing or (2) acidification with hydrochloric acid then water washing. Alkali pretreatment improved the enzymatic digestibility of rice straw by increasing the cellulose accessibility to cellulases. However, acidification after pretreatment reversed the gains in cellulose accessibility to cellulases and enzymatic digestibility due to precipitation of solubilized compounds. Surface composition analyses by ToF-SIMS confirmed a reduction in surface lignin by pretreatment and water washing, and suggested that acidification precipitated a chemically modified form of lignin on the surfaces of rice straw. The spin-spin relaxation times (T-2) of the samples indicated increased porosity in alkali pretreated rice straw. The acidified pretreated rice straw had reduced amounts of water in the longer T-2 proton pools associated with water in the pores of the biomass likely due to back-filling by the precipitated components. (C) 2014 Elsevier Ltd. All rights reserved.