Isocyanates have been in use for some time as cross-linking agents for polyester and polyether polymers. These form three-dimensional network polymers on crosslinking and are familiar commercial materials. This cross-linking system has the merit that it is possible to produce a wide range of materials from a given isocyanate or polyester by varying the reaction proportions, catalysts and so on. In addition, these materials possess a useful range of physical properties. In the present work the chemical basis of cross-linking epoxide resins with isocyanates has been investigated. Previous work on the subject has been largely empirical and though certain workers have produced cross-linked polymers by this means, the precise nature of the cross-linking process has not been established. As an initial step it was decided to investigate the nature of the link between the two types of compound, using monofunctional model compounds. These were phenyl glycidyl ether (2-3 epoxy propyl phenyl ether), and phenyl isocyanate. Phenyl glycidyl ether resembles in structure the active part of the epoxide resin molecule, and phenyl isocyanate is similar to toluene di-isocyanate, which is frequently used in isocyanate cross-linking mechanisms. A reaction has been found to take place giving a crystalline material which is a one-to-one addition compound of the two reagents, together with a red oil, which may be a mixture of alkyl anilides. The structure of the crystalline material has not been fully proved, but is believed to be a 2-oxazolidone.