The substraterecognition of Candida antarctica lipase B (CAL-B) in the acylation of nucleosides was revealed through rational substrate engineering for the first time. CAL-B displayed lower activities and excellent 5-regioselectivities (94 to 99 %) in the acylation of ribonucleosides 1f–1j as compared to those in the acylation of 2-deoxynucleosides 1a– 1e. The excellent regioselectivities might be