The kinetics of the formation of ozone was studied by using pulse radiolysis coupled with time-resolved UV absorption at 275 nm and at T=294.9¿0.6 K. The rate constant for the formation of ozone 16O16O16O in argon was determined to be k3a=(3.38¿0.04)¿10-34 cm6 molecule-2 s-1. The rate constants for the reactions 18O+16O16O(k3b), 16O+16O18O(k3c), 16O+18O18O(k3d), 18O+16O18O(k3e), and 18O+18O18O(k3f) were studied, and the following parameters were determined: (k3b+k3d)/(2k3a)=(1.184¿0.037), (k3c+k3e)/(2k3a)=(1.155¿0.062), and k3f/k3a=(0.977¿0.021). The values for (k3b+k3d)/(2k3a) and (k3c+k3e)/(2k3a) obtained here are equal to the values derived from the product studies and the recently reported relative rate study but higher than the reported values for (k3b+k3d)/(2k3a) and (k3c+k3e)/(2k3a) obtained by using CO2 as a third body. The parameter k3f/k3a=(0.977¿0.021) is lower than the value of k3f/k3a obtained by using CO2 as a third body and the value derived from the product studies. These different values of k3f may be partly due to changes in third body efficiency or due to resonance interactions between the excited ozone molecules and the third body. The absolute measurements reported here together with literature data suggest that the nature of the third body is an important factor in controlling the enhancements of the rate constants for ozone formation and that asymmetry of neither ozone nor dioxygen ensure a fast ozone formation rate. ¿ 1998 American Geophysical Union