The potential energy surface of the complex formed between Ar and PH3 is investigated by ab initio methods. In the equilibrium geometry, the Ar atom lies 3.75 Å from the P center; the Ar··P vector makes an angle of 75° with the C3 symmetry axis of PH3. The SCF portion of the interaction energy is repulsive but is counteracted by an attractive MP2 contribution. The SCF term is more sensitive to the relative orientation of the two subunits and is hence responsible for the angular features of the equilibrium structure. The binding energy of Ar··PH3 is calculated to be about 70 cm−1 at the correlated level after correction for basis set superposition error.