Irradiation with u.v. light of a methanolic solution of β-D-glucopyranosyl (1), α-D-mannopyranosyl (3), or β-D-galactopyranosyl (4) azide affords in good yield the corresponding next-lower aldose. In the case of β-maltosyl (6), β-D-ribofuranosyl (7), or α-L-arabinopyranosyl (8) azide, there was observed the formation of an intermediate which, on standing in the dark, reverts back to starting material.