It is found that the relative probabilities for the 0 I vibrational excitation in collisions of oxygen isotope O-a atom approaching (OO)-O-b-O-c molecule along a path which has essentially the same geometry as the ozone dissociation path in the asymptotic region coincide quite well with the observed relative formation rates of the (OOO)-O-a-O-b-O-c isotopomers formed in O-a + (OO)-O-b-O-c collisions. It may thus be concluded that isotope selectivity in ozone formation arises primarily from the transfer of the excess kinetic energy of the relative motion into the 0, vibration which produces a vibrationally excited O-3(*) and not from the subsequent collisional deactivation and stabilization through the bath gas. This is in accord with the conclusions arrived at in the recent experimental studies of third-body dependence of ozone formation rates. Further arguments about third-body effects are presented in support of the model. (C) 2002 Published by Elsevier Science B.V.