A technique based on the principle of coupling an evanescent wave via a prism into a semiconductor thin him has been used to determine simultaneously the composition and the thickness of II-VI semiconductor ternary alloys. This approach, which determines the indices of refraction n with high precision (at least 0.1%), and also concurrently determines the epilayer thicknesses with an uncertainty of less than 0.5%, has been applied to a series of molecular-beam epitaxy grown ternary alloy families; Zn1-xCdxSe, Zn1-xMgxSe, Zn1-xBxSe, Zn1-xMnxSe, and ZnSe1-xTex. The composition determined by x-ray measurements allow one to generate a calibration between it and the alloy composition, which is used subsequently to derive the alloy composition of the ternary specimen by measuring only its value of ra. Since the: prism coupler method also determines the thickness of the film, the growth rates are also obtained concurrently.