Organic acids are envisaged as alternative catalysts to strong mineral acids, in pre-treatment of lignocellulosic biomass for anaerobic digestion (AD). To evaluate this hypothesis, an untreated control and four pre-treatments (25 degrees C for 24 h) involving two levels of maleic acid (34.8 and 69.6 kg m(-3)), alone and combined with sulphuric acid (4 kg m(-3)), were studied in three agricultural substrates: Arundo (aka giant reed), Barley straw and B133 fibre sorghum. Methane production was assessed in a batch AD assay (35 degrees C for 51 days) with 4 g L-1 of volatile solid (VS) load. Fibre composition and structure were investigated through chemical analysis and Fourier transform infrared (FTIR) spectrometry. Arundo and B133 that were the most and least recalcitrant substrate, respectively, staged the highest and lowest increase in methane with high maleic acid: +62% over 218 cm(3) g(-1) of VS in untreated Arundo; +36% over 284 cm(3) g(-1) of VS in untreated B133. Barley straw showed an intermediate behaviour (+41% over 269 cm(3) g(-1) of VS). H2SO4 addition to maleic acid did not improve CH4 output. The large increase in methane yield determined by pre-treatments was reflected in the concurrent decrease of fibre (between 14 and 39% depending on fibrous component). Based on FTIR spectra, bands assigned to hemicellulose and cellulose displayed lower absorbance after pre-treatment, supporting the hypothesis of solubilisation of structural carbohydrates and change in fibre structure. Hence, maleic acid was shown a suitable catalyst to improve biodegradability of ligno-cellulosic biomass, especially in recalcitrant substrates as Arundo. (C) 2015 Elsevier Ltd. All rights reserved.