We report the observation of a heteronuclear reverse energy pooling process Cs(9D)+Na(3S)→Cs(6P)+Na(3P).The. State is populated in a stepwise excitation process by a dye-laser beam. A double-modulation technique is used to select the fluorescence contribution at the Na(3PJ) exit channels only due to the above reaction. The ground-state sodium atom density was obtained by measuring the wing absorption coefficient. The measured density and fluorescence intensities are used to obtain the reverse energy-pooling rate coefficient. The cross section related to the rate coefficient is (4.3 ± 1.4) × 10-14 cm2.