Molecular dynamics simulations of thermodynamic properties and order parameters are reported for mixtures of argon and nitrogen adsorbed as a monolayer in registry with the graphitic basal plane. Two concentrations were treated: 5·41 and 10·42 mole per cent Ar, using initial argon positions in the layer selected randomly. Although the calculated properties of these phases show some scatter from run to run (due to differences in the initial Ar atom configurations), the results showed quite clearly that the in-plane orientational order-disorder transition progressed to a lower temperature, became broader and involved smaller energy changes as the pure N2layer was diluted with argon.