The pretreatment of softwood with sulfuric acid impregnation in the production of ethanol, based on enzymatic hydrolysis, has been investigated. The parameters investigated were : H2SO4 concentration (0.5-4.4% w/w liquid), temperature (180-240 degrees C), and residence time (1-20 minutes). The combined severity (log Ro-pH) was used to combine the parameters into a single reaction ordinate. The highest yields of fermentable sugars, i.e., glucose and mannose, were obtained at a combined severity of 3. At this severity, however, the fermentability declined and the ethanol yield decreased. In a comparison with previous results, SO2 impregnation was found to be preferable, since it resulted in approximately the same sugar yields, but better fermentability.