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Highlights • Microwave-based Organosolv pretreatment is efficient for wood biomass fractionation. • Treatment temperature, ethanol-water ratio and H 2 SO 4 addition were studied. • Optimum conditions are 175 °C, 60:40 v/v ethanol-water and 0.25% w/w H 2 SO 4. • Cellulose recovery and purity reached 80% and 70%, with 45% lignin recovery. • The production of fermentation inhibitors was minimized. Abstract A mixture of sawmill feedstocks comprising four softwood species (fir, spruce, Scots pine, and Douglas fir) was subjected to a microwave-assisted Organosolv pretreatment. The influence of H 2 SO 4 as a catalyst, ethanol-water ratio as the solvent, and temperature on wood fractionation, cellulose yield and purity, lignin recovery, and inhibitor formation were investigated. Minute addition of H 2 SO 4 (0.25% w/w) was found to enhance lignin extraction and hemicellulose hydrolysis, thus providing cellulose yield and purity above 68% ± 2% and 61% ± 3%, respectively. The increase in H 2 SO 4 up to 1% and temperature up to 190 °C, impaired cellulose yields a result of intense hydrolysis, thus degrading cellulose and hemicellulose. Conversely, the increase in ethanol-water ratio enhanced delignification without cellulose degradation. Similarly, inhibitor formation increased with pretreatment severity (H 2 SO 4 content and temperature), but was mitigated by higher ethanol-water ratio. Finally, under best conditions (60:40 ethanol-water, 175 °C, 0.25% H 2 SO 4), cellulose yield and purity reached 82% ± 3% and 71% ± 3%, respectively. [ABSTRACT FROM AUTHOR]