Preferentially oriented perovskite-structured Na0.5K0.5NbO3 (NKN) thin films have been deposited on hexagonal Al2O3(011_2) substrates using rf magnetron sputtering of a stoichiometric, high-density, ceramic target. Structural and film surface properties were measured using x-ray diffraction and atomic force microscopy, respectively. Optical and waveguiding properties were characterized using a prism-coupling technique. We observed sharp and distinguishable TM and TE propagation modes and measured the refractive index of NKN thin films of different thicknesses. The ordinary and extraordinary refractive indices were calculated to be no=2.247±0.002 and ne=2.216±0.002 for a 2.0-μm-thick film at 632.8 nm. This implies a birefringence Δn=ne-no=-0.031±0.002 in the film. These first results show the potential use of rf-sputtered NKN films as an electro-optical active material. © 2003 American Institute of Physics.