Molecular-dynamics calculations have been carried out for a model of argon physisorbed on the (100) surface of magnesium oxide. The commensurate solid at full monolayer coverage is shown to melt by second-layer promotion. At lower coverages, the system is found to evolve spontaneously to a latticefluid state in which motion of the argon atoms is largely confined to the (10) channels between magnesium sites. Comparison is made with the results of recent experiments and with the predictions of the jump-diffusion model of Chudley and Elliott.