The structure and ground state dynamics of the atom-diatom dimer interaction between Ar and HI has been investigated by microwave and near infrared supersonic jet spectroscopy. Ab initio molecular orbital calculations were used to provide greater insight into the nature of the interaction. The ground state is shown to be in the isomeric form Ar-IH with R-cm=3.9975(1) A, theta=149.33(1)degrees for the normal isotopomer and R-cm=3.9483(1) Angstrom, theta=157.11(1)degrees for Ar-ID. The potential surface from an ab initio molecular orbital calculation was scaled and shifted to yield a nonlinear least-squares fit of the rovibrational state energies to the experimental data. The ground state potential energy surface obtained in this manner has a barrier between the Ar-IH and Ar-HI isomers of 88.5 cm(-1) with respect to the global minimum. Such calculations are also used to predict the presence of localized states in the secondary minimum associated with isomers Ar-HI and Ar-DI. Attempts to experimentally identify transitions associated with the latter were unsuccessful. The ground state, Ar-IH isomeric structure, contrasts with the corresponding ground state of the other members of the homologous series Ar-HX (X=F, Cl, and Br) in which the Ar is bound to the proton.