Annealed proton-exchange (APE) waveguides in lithium niobate are widely used in integrated photonics for navigation, telecommunications and electric field sensors. In this paper, we analyze the characteristics of planar APE waveguides in new mixed single crystals of lithium niobate-tantalate solid solution by various methods (prism coupling, XRD and IR spectroscopy). The elemental composition (ratio Nb/Ta) of Z-cut mixed lithium niobate-tantalate samples gives a good uniform distribution over the all-surface area by X-ray fluorescence method. APE waveguides in mixed lithium niobate-tantalate single crystals are characterized by a lower value of refractive index increment due to Ta atoms and higher proton diffusion coefficients due to lattice disordering. High diffusion coefficients provide a deeper APE waveguide layer and rapid recovery of the crystal lattice during post-exchange annealing. These results expand the understanding of the proton exchange process in mixed lithium niobate-tantalate solid solutions for the creation of single-mode APE waveguides.