The composition, structure, and optical properties of thin-film ZnS1-xSex semiconductor alloys were studied by x-ray photoemission spectroscopy, x-ray diffraction, and optical transmission. Nonequilibrium growth of the ternary II-VI semiconductor alloys was accomplished using molecular beam epitaxy with simultaneous electron cyclotron resonance H2S plasma activation to form the ZnS1-xSex alloys. Photoemission measurements were used to identify changes in bulk composition and valence band electronic structure as well as changes in the Zn 3 d, Se 3 d, and S 2p core lines. X-ray diffraction measurements revealed that these films contain the cubic phase and that the lattice constant decreases with sulfur content. Correlation of these results with growth parameters indicates that substrate temperature, gas flow, and plasma power determine the ZnS1-xSex alloy composition and structure. In addition, optical transmission measurements showed that substitution of S into the ZnSe lattice shifted die cutoff from 460 to 380 nm.