The results of an investigation of dielectric properties of perovskite-type single crystals with solid solutions of potassium-lithium tantalate K1-xLixTaO3 are presented. Particular attention is focused on two selected concentrations of x, 2% and 4.3%, which are representative for the samples with lower and higher concentration of lithium ions, respectively. While a peak in dielectric losses, as observed at higher temperature, has just been revealed for low Li content, it becomes well developed for high lithium concentration. In such crystals there exist noticeable oxygen vacancies, and additional defects associated with lithium ions have a great influence on the dielectric losses in low temperature scope. The consideration pertains to the temperature dependence of the activation energy of the possible polaronic state in the context of lithium ion content.