The interaction of alcohols in the hydrolysis of aryl beta-D-glucopyranosides and aryl beta-D-xylopyranosides by beta-D-glucosidase (beta-D-glucoside glucohydrolase, EC 3.2.1.21) from Stachybotrys atra has been investigated. The results constitute support for the presence of a glycosyl-enzyme intermediate, formed during the first step (glycosylation) of the proposed two-step mechanism. Transfer of the glycosyl group to an alcohol, with the formation of an alkyl glycopyranoside, can take place in parallel to the transfer to a water molecule (second or deglycosylation step). The alcohol binds to the free enzyme and to the glycosyl-enzyme intermediate. The glycosyl-enzyme-alcohol complex undergoes hydrolysis in addition to the alcoholysis. For aryl beta-D-glucopyranosides the deglycosylation step is rate-limiting. For aryl beta-D-xylopyranosides two kinds of substrate behaviour can be observed. Depending on the substituent group on the phenyl ring, either both steps are rate-controlling or the first step is rate-limiting. Electron-withdrawing substituents increase the rate at which the substrate aglycon group is released.