This paper presents experimental p-rho-T measurements for nitrogen + methane mixtures containing approximately 0.25, 0.50, and 0.75 mole fraction methane at T = 300, 350, 400, and 470 K with p = 10-137 MPa. Methane, nitrogen, and argon are used as calibration fluids, and measurements on carbon dioxide and ethane verify the calibration methodology. An error contribution analysis indicates that the expanded uncertainty (95% confidence limit) is 0.1 kgm(-3) for densities lower than 200 kg-m(-3) and 0.0005.rho for higher densities. The GERG-2008 equation of state describes the measurements for all three mixtures to within +/- 0.0015.rho with the largest deviations occurring at the highest pressures.