In order to insert a description of the thermodynamic properties of crude oils and natural gases into a petroleum model developed to improve reservoir location and evaluation, one important step is to obtain liquid-vapour equilibrium data. Because of the complexity of the analytical representation of reservoir fluids, we have selected a set of ’key components’ with special attention given to heavy hydrocarbons with ramified or polycyclic structures. Experimental determinations of bubble points and liquid phase densities have been performed along isotherms for binary (methane-key component) systems, ternary systems and synthetic mixtures. These data extend published information to heavy hydrocarbons and high pressures.