ABSTRACT The thermodynamic properties of binary hard-core Lennard-Jones fluid mixtures have been obtained by Monte Carlo simulation for different compositions, maximum well depths, hard-core diameter ratios and temperatures. These data are used in combination with a one-fluid theory based on perturbation theory to analyse the performance of several theoretically based mixing rules proposed in the literature. Two new theoretically based density- and temperature-dependent mixing rules are proposed. The new mixing rules provide better agreement with simulations than those previously proposed, especially for very asymmetric mixtures at low temperatures.