A new apparatus for the measurement of the phase behaviour and pVT properties of fluid mixtures has been developed. The instrument is based on the isochoric method, can operate at temperatures up to 773 K and at pressures up to 60 MPa, yields absolute results and is fully automated. A typical isochoric run yields pressures in both the single- and two-phase regions from which the dew- or bubble-pressure of the mixture may be determined with high precision. Densities of the homogeneous fluid may be obtained with an overall uncertainty of less than +/-0.1%. In the present work, we have measured bubble pressures and homogeneous-fluid densities in the following systems: (CH4 + n-C4H10), (CH4 + n-C7H14) and (CH4 + n-C16H34). The experimental results are compared with several well-known thermodynamic models each containing binary parameters which we have fitted to the experimental data.