Results of TG and DTA studies as well as an analysis of the liberated gas products have led us to recognize differences in the mechanisms of transformations taking place in the systems NH4ReO4/Al2O3-SiO2 (25 wt% SiO2 and NH4ReO4/Al2O3 containing 1.1, 3.3 and 3.3, 9.9, 17.8 wt% NH4ReO4. Thermal decomposition of NH4ReO4 on the supports used begins with release of ammonia, which is strongly bound with the surface in the system of 3.3 wt% NH4ReO4/Al2O3, and undergoes oxidation to nitrogen oxides in the air atmosphere. In the other systems studied, the process of ammonia release starts already at 473 K and ammonia does not get oxidized. Moreover, it has been established that ammonia perrhenate supported on the surface of Al2O3-SiO2 in the amount of 1.1 or 3.3 wt% undergoes partial thermal decomposition to ReO2 which is further oxidized in the air atmosphere. As follows from the thermal studies as well as the measurements of activity in a reaction of 1-hexene metathesis, the active centres of the reaction of olefin metathesis are formed on the surface of the studied systems after their calcination at 473 K.