Slab waveguides were constructed in K1-x-LixTa 1-yNbyO3 crystals by the implantation of 12C+4 ions at 30 MeV and 16O+5 ions at 30 and 40 MeV. The waveguides were characterized by a prism coupler setup. A refractive index drop of 10.9% was observed in a layer formed by the implantation of 16O+5 ions at 30 MeV. The carbon-implanted waveguides were found to be thermally stable after annealing at 450 °C A semiempirical formula for predicting the change in the refractive index given the parameters of the implantation process was developed. It is argued that the combination of the basic implantation process with the semiempirical formula can be developed to become a generic method for constructing complex electro-optic circuits with a wave-guided architecture.