Potential-energy surfaces have been calculated over the full range of nuclear configurations for the H + Cl2 reaction using the DIM (diatomics-in-molecules) method with 14 polyatomic basis functions arising from the atomic states H(2S), Cl(2Pu), H+(1S) and Cl–(1S). The lowest 2A′ potential-energy surface favours reaction in the collinear configuration with an activation energy in good agreement with the experimental value and a higher barrier to reaction in bent configurations. Comparison with analogous calculations on H + F2 shows a lower activation energy for H + Cl2, but otherwise the potential-energy surfaces have very similar topography favouring the collinear configuration for both reactions.