The main purpose of this work was the determination of the composition of a KY1−x−yGdxLuy(WO4)2 layer grown by liquid phase epitaxy (LPE) technique on KY(WO4)2 substrate having enough refractive index contrast and appropriate lattice mismatch (fhkl) to act as planar waveguides. To achieve this we have grown KY1−x−yGdxLuy(WO4)2 single crystals by the flux method and characterized them in terms of chemical composition, structure and refractive indices. A high quality epitaxial layer of KY0.58Gd0.22Lu0.20(WO4)2 with a thickness of 10 μm was grown on a b oriented KY(WO4)2 substrate by LPE. At λ = 632.8 nm this planar waveguide can support five and four guided modes along the Ng and Nm directions, respectively and five TM modes along Np direction.