Auger electron spectroscopy (AES), x‐ray photoelectron spectroscopy (XPS), and electron microprobe analysis (EMA) were applied to investigate the chemical composition, impurities, and the homogeneity of thin para‐ and ferroelectric epitaxial KTa1-xNbxO3 (KTN) layers grown by liquid‐phase epitaxy using a KTN‐KF flux. AES indicated no Ta/Nb fluctuations within the layer plane and in the growth direction. XPS and EMA determined a F incorporation of 0.8–0.9 mol % for as‐grown and 0.6 mol % for annealed samples. Valence analyses with XPS and electron paramagnetic resonance (EPR) revealed only the regular valence state for the B atoms of a (I,V)ABO3 perovskite lattice. An unexpected splitting of the K 2p doublet peak was found for as‐grown layers. Structural defects observed by optical microscopy and x‐ray topography were attributed to lattice relaxation effects. Optical waveguiding could be achieved for para‐ and ferroelectric layers.