Physiochemical properties of native and dilute acid pretreated (0.6% H2SO4, 10 min, and either 170 degrees C or 180 degrees C) poplar were investigated before and during simultaneous saccharification and fermentation (SSF). SSF duration was 5 days and employed Trichoderma reesei cellulases and Saccharomyces cerevisiae fermentation. Chemical composition (glucan, xylan, lignin), enzyme-accessible surface area (based on solute exclusion), crystallinity index, particle size distribution, particle shape, and enzyme adsorption (cellulase, beta-glucosidase) were compared to cellulose conversion. Cellulose conversion varied from 8% for native poplar to 78% for the 180 degrees C-pretreated poplar. The physicochemical properties of native poplar changed little during SSF. In contrast, the physicochemical properties of the 180 degrees C-pretreated feedstock changed markedly. Enzyme-accessible surface area and beta-glucosidase adsorption increased by 83% and 65%, respectively, as cellulose was removed from the feedstock. Crystallinity index and particle size (large fraction) decreased by 65% and 93%, respectively. Cellulase adsorption per unit weight increased initially (+45%) followed by a slight decrease (-13%). The same trends were observed, although to a lesser extent, for 170 degrees C-pretreated feedstock.