Abstract
Monte Carlo results are presented for the temperature dependence of the tracer diffusion coefficient in the square planar lattice gas with nearest-neighbour interactions at a concentration of 0·5. Below the order/disorder temperature the Arrhenius plot is linear. Above the order/disorder temperature the plot is curved towards lower activation energies. Results are also presented for the temperature dependence of the ratio of the tracer correlation factor to physical correlation factor-the Haven ratio. A remarkable discontinuity is found above the order/disorder temperature.