A dynamic method of chemical counting of active centers of alkene metathesis is proposed and applied to study MoO3/Al2O3 and MoO3/Al2O3-SnMe4 systems. For the MoO3/Al2O3 catalyst, the estimated number of the active sites is less than 1% of the total number of the Mo atoms. MoO3/Al2O3-SnMe4 catalysts have a larger percentage of active centers, compared with the system not treated with SnMe4. This increase of the active sites number approximately corresponds to improvement of the metathesis activity of the MoO3/Al2O3-SnMe4 catalyst.