The p-rho-T behaviors of four lean synthetic natural gas mixtures were measured along isotherms of 250 K, 350 K, and 450 K with pressures to 37 MPa with a two-sinker, magnetic-suspension densimeter. The gravimetrically prepared mixtures have seven to nine components (methane to pentane). They are all nominally 0.90 mole fraction methane, and they differ primarily in the amount of pentane and isopentane and the presence or absence of nitrogen and carbon dioxide. The analysis for density accounts for the force transmission error in the magnetic suspension coupling (MSC) of the densimeter and the magnetic effects of the fluid. These data are compared to the GERG 2004 and AGA-8 models for natural gas mixtures. The densities calculated by both the models are generally in very good agreement with the data, with average deviations less than 0.05 %, at the higher temperatures (T = 350 K and T = 450 K) and lower pressures (p < 15 MPa). But at higher pressures and lower temperatures, the average deviations are substantially larger, ranging up to 0.28 % and 0.93 % for the GERG 2004 and AGA-8 models, respectively, for T = 250 K and p > 15 MPa for one or more of the mixtures. Additional data on these same mixtures were measured by Texas A&M University and are reported in a previous paper.