A method to determine the lattice parameters of proton-exchanged (PE) lithium tantalate solid solutions by examining the PE processes on rotated (0kl) cuts is proposed. It has been shown that, depending on exchange and annealing conditions and crystal cuts, the five phases α-, β-, γ-, δ- and =η-HxLi1-x TaO3 can exist in single crystal layers. The phase boundaries depend on plate orientations and are determined only by the value of molar elastic energy. The relationships between the lattice parameters and the index change in α-, β-, γ- and δ-unstrained HxLi1-x TaO3 solid solutions has been experimentally determined. The regularities of proton exchange in lithium tantalate crystals under different exchange and annealing conditions are studied and various specific features are explained.