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Results of 'rG and DTA studies as well as an analysis of the liberated gas products have led us to reeogniz~ differences in the mechanisms of transformations taking place in the systems NH4ReO4/A1203-SiO2 (25 wt% SiO2 and NH4ReOJAI203 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/AIzO3, 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 A1203-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 reaetion of 1-hexene metathesis, the active eentres of the reaction of olefin metathesis are formed on the surface of the studied systems after their calcination at 473 K.