Hydrothermal pretreatment improves bioconversion of lignocellulose, but the effects of different acid catalysts are poorly understood. The effects of sulfuric acid (SA) and sulfur dioxide (SD) in continuous steam pretreatment of wood of Norway spruce were compared in the temperature range 195°C-215°C. The inhibitory effects of the pretreatment liquid on cellulolytic enzymes and Saccharomyces cerevisiae yeast were higher for SD- than for SA-pretreated material, and the inhibitory effects increased with increasing pretreatment temperature. However, the susceptibility to cellulolytic enzymes of wood pretreated with SD was 2.0-2.9 times higher than that of wood pretreated with SA at the same temperature. Data conclusively show that the superior convertibility of SD-pretreated material was not due to inhibition phenomena but rather to the greater capability of the SD pretreatment to reduce the particle size through partial delignification and cellulose degradation. Particle size was shown to be correlated with enzymatic digestibility (R2 0.97-0.98).