Rotational rainbow scattering has been measured in the systems K- and Rb-CO2 at collision energies 0.17≦E≦0.56 eV and CMS angles 110°≦ϑ≦170°. The ellipsoidal hard shell anisotropy of the normal (repulsive) potentials is evaluated to be (c−a)=0.67A in K−CO2, (c−a)=0.63A in Rb−CO2, averaged over a slight energy dependence. Below an apparent activation thresholdEa≅0.23 eV, nearly equal in both systems, the scattering largely conforms to that of diatomic molecules, indicating linear, rigid rotor type behaviour as the dominant collisional response of triatomic CO2. AboveEa characteristic, eventually drastic distortions set in. They are probably caused by intermediary charge transfer which activates a complex, coupled vibrational, rotational motion inM+-CO 2 − , altering the outcome of the collisions altogether. The simple model of a partially absorptive, hard shell is used to find that some gross features of the distortions are consistent with distinct properties of the acceptingelectronic orbital 6a1 in CO 2 − .