The influence of relativistic effects on the linear stationary points of the potential energy surfaces of the ClHCl and HCl2 systems is studied. Scalar relativistic effects have little influence on the geometries and the energies at these points. Spin-orbit effects have no influence on the barrier of the forward reaction of H with Cl2, but increase the reaction energy. The total relativistic effect is an increase of the reaction energy by 2.3 kJ mol−1. The barrier for the iso-energetic exchange reaction of HCl with Cl is decreased by 0.5 kJ mol−1 due to the scalar relativistic effects. This small decrease is cancelled by the larger increase of 2.5 kJ mol−1 due to spin-orbit effects, giving a total relativistic effect of 2.0 kJ mol−1.