The aim of this work was to study the effect of the solvent N-methylmorpholine-N-oxide (NMMO) to pretreat Nannochloropsis oculata before the anaerobic digestion process. The results indicated that the pretreatment affects the characteristics of the cell wall, which consequently becomes more susceptible to the microorganisms attack during anaerobic digestion. The methane production was increased by 43% after the pre-treatment, from 238 +/- 6 mL(CH4)/g(VS) until 339 +/- 4 mL(CH4)/g(VS). On the contrary, the methane production from Chlorella vulgaris decreased after the pre-treatment from 251 +/- 4 mL(CH4)/g(VS) to 231 +/- 3 mL(CH4)/g(VS). The failure on the pre-treatment was attributed to the particular characteristics of the substrate in consequence of a previous drying step. (C) 2015 Elsevier Ltd. All rights reserved.