ZnSe-based II-VI ternary alloys, such as ZnBeSe, ZnMgSe, and ZnCdSe, have been shown to be excellent candidates for light-emitting device applications in the short-wavelength visible range. Since design of optical semiconductor devices requires knowledge of refractive indices of all materials involved, we studied refractive indices of Zn1-xBexSe, Zn1-xMgxSe, and Zn1-xCdxSe epitaxial layers. The samples were grown by molecular-beam epitaxy in mole fractions ranges between 0 less than or equal to x less than or equal to 0.81, 0 less than or equal to x less than or equal to 0.59, and 0 less than or equal to x less than or equal to 0.57 for Zn1-xBexSe, Zn1-xMgxSe, and Zn1-xCdxSe, respectively. The alloy concentration x was determined by x-ray diffraction. All samples exhibited very high crystalline quality, even at relatively high values of x. A prism-to-film. coupler technique, which involves optical tunneling via evanescent fields, was employed to accurately determine the indices of refraction. The variation of the refractive index with alloy composition at room temperature was determined at the wavelength of 632.8 nm.