We report a Monte Carlo simulation of the p(2x1)-->p(1x1) phase transition in a monolayer of CO molecules adsorbed on a NaCl(001) surface. In the p(2x1) phase the CO molecules are tilted by 24 degrees from the surface normal and have preferred azimuthal orientations whereas in the p(1x1) phase the molecules are oriented perpendicular to the surface with no preferred azimuthal orientation. The transition temperature is estimated to lie in the range 30-35 K. Multiple domains and phase coexistence are also observed.