Comprehensive (p, p, T) measurements on pure methane (159 values) and pure carbon dioxide (118 values) have been carried out in the temperature range 240 K to 520 K at pressures up to 30 MPa. The measurements were performed by means of a single-sinker densimeter that is based on Archimedes' buoyancy principle. The total relative uncertainty of the measurements in the density p was estimated to be less than or equal to \ (1.5 to 2). 10(-4).rho \ for methane and less than or equal to \ (2 to 3). 10(-4). rho \ for carbon dioxide. The measurements are compared with previous results of other experimentalists and with values calculated from current equations of state. Moreover, previously unpublished results of (p, p, T) measurements on carbon dioxide (29 values) and methane (53 values) are listed in the appendix.