The preparation of benzyl β-cellobioside 2,3,5,2′,3′-pentabenzoate ( 2b) via a 4′,6′-benzylidene acetal in 27% overall yield from cellobiose is described. This pentabenzoate has been utilised for conventional syntheses of the 6′-amino-, 4′-amino-, and 4′,6′-diamino-analogues of lactose, via the 4′,6′-dimesylate. In addition, reaction of the pentabenzoate 2b with sulphuryl chloride initially afforded the 6′-chloro derivative, which was then slowly transformed into a mixture of the 4′,6′,-dichloro-lactoside and an isomer in which the non-reducing ring of the disaccharide had been transformed into a 3′,6′-dideoxy-3′,6′-dichloro-gulopyranoside. The latter probably arose by neighbouring-group participation by the 3′-benzoyloxy group prior to the introduction of the secondary 3′-chloro substituent. The former dichloro compound was subsequently converted into 4′,6′-dideoxy-4′,6′-dichloro-lactose. It was found that the rate of nucleophilic displacement at C-4′ of a cellobioside was much lower than that for the corresponding α-linked disaccharide, maltose. Furthermore, nucleophilic displacement at C-4′ of a lactoside was accompanied by substantial elimination, to give the 4′ene, together with some cleavage of the inter-glycosidic bond. The origins of these effects are discussed.