A chemisorption technique has been developed for the quantitative determination of active centres in Re2O7-Al2O3 metathesis catalysts. Butene-2 and ethylene were used as chemical probes to generate metal-carbene species on the active centres of the catalyst and eventually propylene as product. Catalysts with metal loading from 2% to 8% Re were shown to contain active centres in an amount equivalent to 0.6-2.2% Re/g catalyst. Catalysts prepared in hydrochloric acid medium contain about 5% more active centres than those prepared in aqueous medium, as measured by metathesis of propylene. It was also demonstrated that the surface area of the support could further influence the activity of the catalysts.