steam explosion after sulfur dioxide impregnation of wood chips is an effective method for improving the enzymatic digestibility of cellulose in the softwood pinus radiata. digestibility of pretreated fiber was progressively increased by altering the conditions of steam explosion. with increasing digestibility, there was an observed increase in fiber porosity as measured by the solute exclusion technique. accessible pore volume and accessible surface area to a 5-nm dextran probe positively correlated with both 2- and 24-h digestion yields from pretreated fiber. the increase in accessibility was probably the result of hemicellulose extraction and lignin redistribution. a subsequent loss in accessibility, brought about by structural collapse or further lignin redistribution, resulted in a corresponding loss in digestibility. it appears that steam explosion increases cellulose digestibility in p. radiata by increasing fiber porosity. (author abstract) 33 refs