Thin film waveguides of ferroelectric materials hold great promise for use in active integrated optics devices because of the high optical confinement possible in a thin film structure. KNbO3is an attractive material for active devices because it possesses large nonlinear optical susceptibilities and large electro-optic coefficients. KNbO3films with low optical losses are required to produce efficient devices. Epitaxial films of KNbO3(110) have previously been deposited on single crystal MgO (100) using ion beam sputtering techniques. However, these films contained microstructural defects due to the large lattice mismatch (>4.0%) between KNbO3and MgO which resulted in high optical losses. Recent work has focused on determining the relationships between microstructure and optical loss through the use of lattice matched substrates. Film composition, epitaxial quality and optical properties of KNbO3films deposited on MgO and MgAl2O4have been investigated and are compared.