Although considerable progress has been made in technology for converting lignocellulosic biomass into ethanol, substantial opportunities still exist to reduce production costs. In biomass pretreatment, reducing milling power is a technological improvement that: will substantially lower production costs for ethanol. improving sugar yield from hemicellulose hydrolysis would also reduce ethanol production costs. Thus, it would be desirable to test innovative pretreatment conditions to improve the economics by reducing electrical power of the milling stage and by optimizing pretreatment recovery of hemicellulose, as well as to enhance cellulose hydrolysis. The objective of this study was to evaluate the effect of chip size (2-5, 5-8, and 8-12 mm) on steam-explosion pretreatment (190 and 210 degrees C, 4 and 8 min) of softwood (Pirus pinaster).