Ejection of atoms and molecules from the atmosphere of Mars is caused by energetic O+ pickup-ion bombardment <Luhmann et al., 1992; Jakosky et al. 1994>, a process referred to as atmospheric sputtering. In an atmosphere which is primarily a mix of O and CO2, collisional dissociation of CO2 may have a large effect on the sputtering efficiency <Kass and Yung, 1995, 1996>. In this paper, we calculate dissociation cross sections for O+CO2, O+CO, CO+CO2, and CO2+CO2 collisions in the energy range 20 eV to 1 keV. Semiempirical interaction potentials are used from which cross sections are calculated via classical molecular dynamics. The O+CO2 dissociation cross sections and the energy transfer to the fragments are compared to that calculated using the binary encounter approximation (BEA) Sieveka and Johnson, 1984> and to the dissociation cross section model used by Kass and Yung <1995> in their calculation of atmospheric loss from Mars. The former are useful well above the dissociation threshold energy, whereas the latter significantly overestimate dissociation at all energies. Modifications of the BEA model for atom-molecule collisions and that of Kass and Yung for molecular-molecular collisions are suggested. The cross sections created here will be used in a Monte Carlo calculation of the atmospheric sputtering rate. ¿ 1998 American Geophysical Union