Using two steps excitation of the cesium 8D atomic level in a cesium-potassium vapor mixture, we observed the reverse heteronuclear energy-pooling collision Cs(8D)+K(4S) →Cs(5D)+K(4P) at thermal energy. A double-modulation technique has been used to select the fluorescence contributions at the K(4PJ) exit channels due only to the above reactions. The ground state potassium atom density was measured by the absorption of line from a K hollow-cathode lamp. The measured density and fluorescence intensities have been used to obtain reverse energy-pooling rate coefficients. The contribution to the rate coefficients from other processes is discussed.