We report an extended corresponding states model optimised for accurate prediction of the thermodynamic properties and vapour-liquid equilibria of natural gases and similar mixtures. The corresponding states model uses methane as the reference fluid and employs shape factors for pure components that were reported recently [Fluid Phase Equilib. 204 (2002) 15]. The van der Waals one-fluid model is used for mixtures and, in this paper. we report two alternative temperature- and density-dependent correlations of the binary interaction parameters. Model ECSmixS1 was optimised for 19 binary systems in the wide domain 90 less than or equal to T (K) less than or equal to 670 with p (MPa) less than or equal to 510 and is suitable for the prediction of both liquid- and gas-phase thermodynamic properties and for the solution of vapour-liquid equilibrium problems. Model ECSmixS2 was specialised for increased accuracy in the natural gas 'custody transfer' interval 270 less than or equal to T (K) less than or equal to 330 with p (MPa) less than or equal to 12 and is intended for gas-phase thermodynamic properties only. For mixtures of the major components of natural gas, we obtain with Model ECSmixS1 an overall average absolute deviation (AAD) of 0.12% in calculated densities, an AAD of 0.16% in calculated speeds of sound and an AAD of 1.8% in bubble pressure. With Model ECSmixS2, we obtained improved AADs of 0.03% in density and 0.03% in speed of sound. These results compare very favourably with other commonly used mixture models. The present model may be systematically improved or extended by introducing new or improved correlations of the binary parameters. (C) 2003 Elsevier B.V. All rights reserved.