Abstract: The chemiluminescent reactions of ClO_2 with Ca, Ba atoms have been studied under single collision condition in a beam-gas apparatus. The metastable states of the metal atoms are generated by the use of low valtage DC discharge. The Chemilu-minescence spectra have been observed and the spatial distributions of the intensities of the chemiluminescent emissions have been also measured by OMA-Ⅱ. The total reaction cross sections of metal atoms measured are 0.97 nm, 1.38 nm for Ca(~1S), Ca(~3P) and 1.36 nm, 2.39 nm for Ba(~1S), Ba(~3D) respectively. The intensities of the chemilu-minescence spectra are increased significantly when the metal atoms are excited to their metastable states. In particular, the increase of BaO(A′) is much more than that of BaCl(C) in the reaction of Ba(~3D)+ClO_2. In addition, non-Boltzmann vibrational distributions were found in the electronic excited state A′~1П of the products MO in both reactions by computer simulation. The chemiluminescent cross sections of the transition C~2П→X~2∑~+ of the products MCl are 0.51 nm for Ba(~3D)+ClO_2, 0.31 nm for Ca(~3P)+ClO_2 respectively. The branching ratios of the products MO and MCl were calculated based on the chemiluminescent spectra. A discussion about the mechanisms of these reactions is presented.