The compounds of the composition R,Sn,X,O, which can be prepared by partial hydrolysis of R,SnX, or by synthesis from R,SnX, and R,SnO, have been investigated by lH and l19Sn magnetic resonance spectroscopy, and by vapour-pressure osmometry. It is concluded that in non-polar solvents, these compounds are dimeric and contain 4-and 5-co-ordinate tin, probably in the cyclic structure (IV) or (V).THE exhaustive hydrolysis of dialkyltin(1v) compounds R,SnX, (X = e.g., halogen or carboxylate) under basic conditions gives the dialkyltin oxides R,SnO which are insoluble, amorphous, infusible solids, and are usually regarded as being high polymers. If, however, the reaction is carried out with a deficiency of base, or with a weak base, at least two intermediate hydrolysis products, R,Sn,X,O and R,Sn,X(OH)O, can be isolated. These compounds are usually highly crystalline, soluble, and low-melting, and hence appear not to be highly polymeric. The same two families of compounds can also be made by dissolving the appropriate amount (1 or 3 mol., respectively) of the dialkyltin oxide in a solution of the compound R,SnX, in an inert solvent, and they have frequently been reported as troublesome by-products in the preparation of organotin compounds. This paper describes an investigation of the compounds R,