The compositional control of ZnSySe1 − y has been investigated using photoassisted metalorganic vapor phase epitaxy (MOVPE), using the sources DMZn, DMSe, and DES, and irradiation from a mercury arc lamp. The solid-phase composition is not a simple function of the gas-phase composition, but depends on the absolute mole fractions of the reactants. Nonetheless, the characteristic is sufficiently gradual so that good compositional control is achieved. The photoassisted MOVPE technique has been used to produce ZnSySe1 − y/ZnSe/GaAs (0 0 1) heterostructures, and these structures have been studied using high-resolution X-ray diffraction. Both pseudomorphic and non-pseudomorphic ZnSe buffer layers were used. The ZnSySe1 − y layers were grown with compositions in the range of 0.00 ⩽ y ⩽ 0.11, and were thick enough (∼ 2 μm) to be non-pseudomorphic. Threading dislocation densities in the ZnSySe1 − y layers increase monotonically with the lattice mismatch (relative to the underlying GaAs substrate). The presence of the ZnSe buffer layer does not modify the lattice-matching condition for the minimum threading dislocation density. However, the absolute dislocation densities in the ZnSySe1 − y layers are increased by the presence of a non-pseudomorphic ZnSe buffer, relative to the predictions of the glide model for the View the MathML source single heterostructure. This is due to the increased linear density of misfit dislocations at the View the MathML source interface. For structures with pseudomorphic buffer layers, threading dislocation densities in the ZnSySe1 − y material are less than those predicted by the single heterostructure theory. This is expected due to incomplete lattice relaxation in the ZnSySe1 − y.