Steam explosion was studied as a means to improve the enzymatic digestibility of carbohydrates in Brewer's spent grain, a protein and lipid-rich lignocellulosic by-product of the brewing industry. Having temperature, treatment time and the presence of acid catalyst as variables, a treatment at 200 A degrees C for 10 min without an acid catalyst was found to be the most efficient, dissolving 12.1 % of the dry matter. Mainly oligomeric non-cellulosic glucan and arabinoxylan were dissolved, and the remaining insoluble carbohydrates could be efficiently hydrolysed by an enzyme cocktail (75 % hydrolysis yield). The process also caused partial protein degradation and dissolved over a third of the total nitrogen. Meanwhile, the insoluble protein appeared to become more strongly associated with acid-insoluble lignin. Compositional changes observed in the proteins and carbohydrates were supported by the results of epifluorescence microscopy. The process yielded three chemically different fractions which could serve as biorefinery products or intermediates.