KNbO3, K(Ta,Nb)O3, KTaO3, and Ta2O5thin films have been grown by ion-beam sputter deposition. KNbO3has excellent nonlinear properties for second harmonic generation; however, high optical losses are still characteristic of these films. Several loss mechanisms, such as, high angle grain boundaries, twin domains, interface and surface scattering, and oxygen vacancies can all contribute to the high losses. In order to isolate the various mechanisms, amorphous Ta2O5films, epitaxial cubic KTaO3and tetragonal K(Ta,Nb)O3films were grown on MgO and Al2O3substrates subjected to post-deposition annealing treatments and various oxygen pressure conditions. The optical losses and refractive indices were observed to differ depending on the substrate surface and annealing treatments. Resonant scattering experiments were performed to analyze the oxygen composition. The optical properties of these oxide thin film systems are reported and the breakdown of the loss mechanisms is addressed.