An ab initio MO method has been used to calculate the potential energy surface for the formation of HF when H reacts with ClF. The various transition states possibly related to forming HF were optimized. An IRC calculation starting from the collinear F centered transition state was performed and energies of a series of points with bending configurations were also calculated to determine the direct reaction path leading to HF product. Another IRC calculation starting from the H centered transition state was run to determine the migratory path to forming HF. By doing so, the Polanyi’s assumption that it would involve the direct and a migratory mechanisms for the formation of HF when H reacts with ClF has been verified theoretically.