Measurements of the (ppT) properties of three natural gas mixtures were carried out at temperatures from 265 K to 335 K and at pressures up to 15 MPa using a densimeter consisting mainly of a DMA 512P measuring cell and a DMA 60 evaluating unit (Anton Paar, Graz, Austria). Four isotherms ((253.15, 273.15, 293.15 and 323.15) K) and seven pressure levels from 0.1 MPa to 15 MPa were investigated for each natural gas mixture at The natural gas mixtures were two real densities ranging from 6 kg (.) m(-3) to 212 kg (.) m(-3) gases from transport pipelines systems of natural gas distributors in the Czech Republic. One sample contained natural gas with nitrogen addition. Four equations of state were examined. The relative deviations of calculated density values from the EOS ranged from (-5.8 to 1.8) per cent. The best results were obtained from the AGA8-DC92 EOS, with the relative deviations of calculated density value being within the range (-0.02 to 0.02) per cent.