The rates of many simple chemical reactions of the type AB+C→A+BC are empirically known to be of the form kOF=PZ exp(—EA/kT). On the theory that the reactions take place through the interchange of translational and vibrational energy, a qualitative quantum-mechanical calculation of the rate kOF is made for the simple model of collinear collisions. The empirical values of the activation energy EA are employed to fix the interaction potential for a given reaction. On identifying the quantum-mechanical expression for the rate kOF with the empirical expression above and on using the gas kinetic expression for the mean number of collisions Z, it is possible to calculate the probability factor P. Numerical calculations have been carried out for the two reactions H2+X→HX+H, where X is Cl or Br. In both cases, P is found to be about 8×10-3.