Efficient pretreatment is necessary to make the wood-to-ethanol process more feasible. In this study, chips of different sizes were impregnated with SO(2) and steam-pretreated. Dilute-acid pretreatment together with subsequent enzymatic hydrolysis resulted in solubilization of between 69% and 73% of the fermentable sugars (glucose and mannose) in the raw material for the combinations of impregnation times and chip sizes investigated. Shorter impregnation times resulted in slightly lower mannose yields for the larger chips, probably due to poor diffusion of the catalyst. Small differences in glucose yield after enzymatic hydrolysis showed that the overall glucose yield was slightly higher for the smaller chips, however, whether the increased energy demand and cost of size reduction is compensated for by the higher yield, requires tech no-economical evaluations. (C) 2009 Elsevier Ltd. All rights reserved.