The thermal behavior of pyrochlores composing of Bi_1·5Mg_0.75M_1.5O_6.75 (M = Nb, Ta) (sp. gr. Fd-3m:2) was studied using the high-temperature X-ray powder diffraction in a wide temperature range of 30–1200 °C. At a temperature above 1080 °C both compounds thermally dissociate forming one of the reaction products MgMO₆ (M = Nb, Ta) whose reflexes are traced on X-ray diffraction patterns after cooling the sample down to room temperature. In the case of Bi_1·5Mg_0·75Nb_1·5O_6.75 orthoniobate α-BiNbO₄ forms at about 800–1020 °C as admixture. The thermal expansion analysis of Bi_1·5Mg_0.75M_1.5O_6.75 (M = Nb, Ta) showed that the compounds studied belong to slightly or moderately expanding materials. Thermal expansion for both compounds is isotropic. With a rise in temperature the unit cell parameter a and the thermal expansion coefficient (TEC) are increased uniformly and slightly for M = Ta (Nb): from 10.52822 (10.55325) Å (30 °C) up to 10.59181 (10.62801) Å (1050 °C) and from 3.8 (30 °C) up to 7.4 (8.9) × 10⁻⁶ °С⁻¹ at 800 °C respectively. The average TEC values in the range of 30–800 °C range are 5.6 (6.4) × 10⁻⁶ °C⁻¹ for M = Ta (Nb) respectively. Bi_1.5Mg_0.75Nb_1.5O_6.75 is characterized with the highest thermal expansion that may be related to the longer bond-length of Nb(Bi)–O in the polyhedra.