Some physico-chemical properties and reactivity in their reduction with hydrogen of NiO—Y2O3 mixed oxides prepared in a dry way have been studied using isothermal thermogravimetry in the range of 320–410°C and temperature-programmed reduction. It was found that addition of small amounts of chloride and acetate anions retarded the reduction of nickel oxide and accelerated the reduction of mixed oxides. The presence of oxalate and formate ions manifests itself by a small positive effect. Introduction of platinum activator or heat treatment of the samples in various atmospheres led to a pronounced increase in the reduction rate. The efficiency of the spill-over effect increases with increasing proportion of non-reducible Y2O3. The pre-irradiation of the samples by accelerated electrons and gamma rays at a dose of 1 MGy results in a negative kinetic effect only with the samples containing an excess of nickel oxide.