The three disaccharides lactose, cellobiose, and maltose in the form of their acetylated glycosyl bromides have been reacted with 8-ethoxycarbonyloctanol to provide the 1,2-trans-glycosides. Conventional Koenigs–Knorr and Helferich conditions provided these glycosides in poor yield but silver trifluoromethanesulphonate, N,N-tetramethylurea gave the disaccharide glycosides in 50–60% yield. Use of 2,4,6-trimethylpyridine as proton acceptor provided the corresponding 1,2-orthoacetates in 60–70% yield. These 1,2-orthoesters were rearranged by stannic tetrachloride to the 1,2-trans-glycosides. The isolation of acetylated 8-ethoxycarbonyloctanol from Lewis acid catalysed isomerisation of 1,2-orthoesters and from Koenigs–Knorr reactions in which N,N-tetramethylurea was the proton acceptor is discussed in terms of a reaction mechanism proceeding from glycosyl halide to glycosidic products via a 1,2-orthoester intermediate. This proposal is supported by 1H nmr evidence for the presence of 1,2-orthoacetate in Koenigs–Knorr reaction mixtures, and by 1,2-orthoacetate isomerisation to glycoside by the conjugate acid of N,N-tetramethylurea.