Electro-optical devices based on KTiOPO4 are produced by the enlargement of the refractive index within small areas of a single crystal surface by Rb+/K+ ion exchange. The local introduction of the larger Rb+ ion instead of K+ should generates a need of space, which could be estimated by the comparison of the lattice parameters of fully relaxed K1-xRbxTiOPO4. Therefore, polycrystalline samples in the mixed crystal system K1-xRbxTiOPO4 were prepared. Whereas the lattice parameter c, is nearly independent of the composition parameter x, a(0) and b(0) increase. This anisotropic expansion could be explained by the oxygen environments of the alkali sites. A Rb+/K+ ion exchange on normally used z-cut KTP single crystals should transform the whole increase of the molar volume into the (001) plane. Diffractometric investigations give evidence for a hindrance of such an expansion in the surface of a single crystal, accompanied with inherent stress.