The densities of six mixtures of (methane + n-butane) were determined over the temperature range 316 to 479 K and the pressure range 8.8 to 48 MPa for mole fractions of methane between 0.3458 and 0.5333. The bubble points of the mixtures were also determined from an analysis of experimental isochores in the one- and two-phase regions, The new measurements have been used in conjunction with selected data from the literature to assess the performance of both the volume-translated Peng-Robinson equation of state and the one-fluid corresponding states model over a wide range of states. The relative strengths and weaknesses of the two models are highlighted.