Abstract Cellobiohydrolase I from Trichoderma reesei catalyzes the hydrolysis of methyl β-D-cellotrioside ( K m = 48μM, k cat = 0.7 min −1) with release of the β-cellobiose (retention of configuration). The same enzyme catalyzes the ( trans-hydration of cellobial ( K m = 116 μM, K cat = 1.16 min −1) and lactal ( K m = 135 μM, k cat = 1.35 min −1), presumably with glycosyi oxo-carbonium ion mediation. Protonation of the double bond is from the direction opposite that assumed for methyl β-cellotrioside, but products formed from these prochiral substrates are again of β configuration. Cellobiohydrolase II from the same microrganism hydrolyzes methyl β-D-cellotetraoside ( K m = 4 μM, k cat = 112 min −1) with inversion of configuration to produce α-cellobiose. The other reaction product, methyl β-cellobioside, is in turn partly hydrolysed by Cellobiohydrolase II to form methyl β-D-glucoside and D-glucose, presumably the α-anomer. Reaction with cellobial is too slow to permit unequivocal determination of product configuration, but clear evidence is obtained that protonation occurs from the si-direction, again opposite that assumed for protonating glycosidic substrates. These results add substantially to the growing evidence that individual glycosidases create the anomeric configuration of their reaction products by means that are independent of substrate configuration.