## Abstract

A re-investigation of the recombination of iodine atoms in presence of the inert gases over a wider range of experimental conditions has shown that the simple termolecular rate law -d(I)/dt = k(I)$^{2}$ (M) is not obeyed. For each of the inert gases k, the experimentally determined termolecular rate constant, increases with the ratio (I$_{2}$)/(M), where (I$_{2}$) and (M) are the concentrations of iodine molecules and inert gas molecules respectively. The dependence of k on (I$_{2}$)/(M) was obscured in previous work by the fact that a thermal effect, which results in a lowering of the apparent value of k as recombination proceeds, increases as (I$_{2}$)/(M) increases and compensated for the real increase in k with (I$_{2}$)/(M). Except at low (I$_{2}$)/(M) values, k is a linear function of (I$_{2}$)/(M), the gradient being the same for all five inert gases. A rapid termolecular reaction I+I+I$_{2}$ = I$_{2}$+I$_{2}$ with a rate constant k = 470 $\times $ 10$^{-32}$ ml.$^{2}$ mol.$^{-2}$ s$^{-1}$ is postulated to explain the linear relationships. By extrapolation the values of k$_{M}$ the third-order rate constants for the five inert gases are $ \matrix\format\c\kern.8em&\c\kern.8em&\c\kern.8em&\c\kern.8em&\c\kern.8em&\c \\ M & \text{He} & \text{Ne} & \text{A} & \text{Kr} & \text{Xe} \\ 10^{32}k_{M}(\text{ml}.^{2}\text{mol}.^{-2}\text{s}^{-1}) & 0\cdot 67 & 0\cdot 92 & 1\cdot 84 & 2\cdot 25 & 2\cdot 99 \endmatrix $