High-accuracy equations of state (EOS) are crucial to industrial applications such as natural gas custody transfer and the development of gas processing facilities. The GERG-2008 EOS, which is the ISO standard for calculating the equilibrium of natural gas mixtures, has a stated relative uncertainty in density of 0.1% over the temperature range from (250 to 450) K at pressures up to 35 MPa. However, for the crucial (methane + propane) binary system, most of the density data sets used in the development of the GERG EOS that extend to pressures above 20 MPa have root-mean-square relative density deviations of (0.4 to 0.8)% and maximum relative deviations of up to approximately 3%. In this work, a single-sinker magnetic-suspension densimeter adapted from a commercial sorption apparatus was used to measure the density of two (methane + propane) mixtures along four isotherms at T = (256, 310, 366, and 422) K over the pressure range from (24 to 35) MPa. The binary mixtures were prepared gravimetrically with methane mole fractions of 0.9472 and 0.8924, respectively. The titanium sinker's volume (V approximate to 3.17 cm(3)) was determined at each temperature as a function of pressure using pure methane and the reference equation of state for this pure fluid, which has a relative expanded uncertainty of 0.03%. The relative combined expanded uncertainty (k = 2) in mixture density measurements ranged from (0.11 to 0.24)%, including the uncertainty in composition. The measured binary mixture densities had relative deviations from those calculated with the GERG-2008 EOS of less than 0.14%. When considered together with other recently measured high-accuracy density data for this binary system, these results help prioritize the nature of future improvements needed for the EOS.