Measurements of the (p, rho, T) properties of five natural gas mixtures were carried out at temperatures from (225 to 350) K at pressures to 35 MPa using an isochoric gas expansion apparatus. Ten isochores have been determined for each mixture at densities ranging from 1 mol . dm(-3) to 16 mol . dm(-3). The gas samples were prepared gravimetrically from pure gases of verified research-grade purity. The uncertainty of the density measurements arises primarily from uncertainties involved in the determination of the experimental cell volume and of the amount of substance contained in the experimental volume. When all components of experimental uncertainty arising from random effects are considered, the expanded uncertainty (comprising 95 per cent confidence interval) of the density measurements is estimated to be 0.044 per cent.