Low-frequency dielectric response of air- and oxygen-sintered ceramics with the composition 0.9BaTiO(3)-0.1La(Mg1/2Ti1/2) O-3 (0.9BT-0.1LMT) has been studied in the temperature range of 12-550 K. In comparison with pure BT, in 0.9BT-0.1LMT the dielectric permittivity maximum is shifted by almost 300 K toward lower temperatures. Both real and imaginary parts of dielectric permittivity of the solid solution, in the range 12-150 K, show a strong frequency-dependent behavior, which is typical of relaxors. On the basis of the model of exponential cluster size distribution and the Cole-Cole equation, the degree of interaction between the polar clusters was estimated. It was shown that the oxygen vacancies arising during sintering at high temperatures did not affect noticeably the relaxor properties of the material. The role of heterovalent La3+/Ba2+ and Mg2+/Ti4+ substitutions in the relaxor behavior formation is discussed.