Thin films of ZnSe on pseudomorphic and partially relaxed (In,Ga)P buffer layers (on GaAs) have been examined; both epilayers were grown by gas source molecular beam epitaxy. The ZnSe layers were grown using elemental Zn and thermally decomposed H2Se. The II-VI nucleation occurred following an ex situ transfer in air of the As-passivated (In,Ga)P buffer layers, grown using In, Ga, and cracked PH3. Microstructural characterization of the II-VI/III-V heterostructures was performed with high-resolution x-ray diffraction and transmission electron microscopy. The (511) reflection of the x-ray rocking curves was used to measure the residual strain in the ZnSe/(In,Ga)P/GaAs structures, and to determine the alloy composition of the (In,Ga)P. The (400) reflection of the x-ray diffraction rocking curves indicated peaks having full width at half-maximum of 130 and 18 arcsec for the relaxed ZnSe on thin (1 mum) pseudomorphic (In,Ga)P buffer layers, respectively. Transmission electron microscopy confirmed the relaxed or pseudomorphic nature of each heterolayer. Thicker (>4 mum) In0.52Ga0.48P buffer layers were still not completely relaxed, and exhibited a residual lattice mismatch of approximately 0.11% between the ZnSe layer and the In0.52Ga0A8P buffer layer. Relatively thick (1-2 mum) ZnSe films had surface morphologies that were featureless when examined by Nomarski microscopy and scanning electron microscopy. Low-temperature photoluminescence spectra originating from the ZnSe films on the partially relaxed (In,Ga)P buffer layers were dominated by donor-bound and free-excitonic features with each exhibiting nearly the same intensity. Luminescence from extended defects (the Y0 and I(upsilon) transitions) within the ZnSe layer, and luminescence originating from the (In,Ga)P buffer layers, were also observed.