Diffusion coefficients are measured for trace amounts of CO and free atoms of Br, I and Tl in the burnt gases of eight pre-mixed laminar flames of H2, O2 and N2, covering the temperature range 1900 to 2520 K at atmospheric pressure. The measured diffusion coefficients are interpreted in terms of both the Lennard-Jones (12 : 6) potential function and a totally repulsive inverse power function describing interactions between the diffusing and flame species. We conclude that the Lennard-Jones function fits the observations better than the purely repulsive one. Values of the Lennard-Jones parameters for each diffusing species are presented, together with the constants characterising repulsive forces.