The microwave dielectric properties of rare-earth titanium niobates can be tailored by preparing solid-solution phases between RETiNbO6 [RE = Pr, Nd, Sm] and RE'TiNbO6 [RE' = Gd, Dy, Y] microwave dielectric ceramics. Dielectric ceramics based on solid-solution phases RE1-xREx'TiNbO6 are prepared by the conventional solid-state ceramic route for different values of x. The ceramic samples are characterized by XRD, SEM, and microwave methods. Ceramics based on RETiNbO6 belonging to aeschynite group show a positive value of tau(f), and those based on RE'TiNbO6 belonging to the euxenite group have a negative value Of tau(f), The solid-solution phases between aeschynites and euxenites show intermediate dielectric constant and tau(f) values. The results indicate the possibility of tailoring the microwave dielectric properties by varying the composition of solid-solution phases. The range of solid solubility of euxenites in aeschynites and aeschynites in euxenites are different for different rare-earth ions. The ceramics have low sinterability and a high dielectric loss factor near the vicinity of aeschynite to euxenite transition. The value of dielectric constant changes abruptly close to the transition point. The abrupt variation in microwave dielectric properties indicate a first-order phase transition at x = 0.86 in Pr1-xGdxTiNbO6, x = 0.48 in Nd1-xDyxTiNbO6 and x = 0.27 in Sm1-xYxTiNbO6 ceramics. The dielectric constant and the sign of tau(f) of the solid-solution phases are found to depend on the average ionic radius of the rare-earth ions in RE1-xREx'TiNbO6. The euxenite to aeschynite phase transition occurs at an average ionic radius of 0.945 Angstrom in all three systems of solid-solution phases.