Comprehensive (p,ρ,T) measurements on two gas mixtures of (0.9CH 4 +0.1N 2 ) and (0.8CH 4 +0.2N 2 ) have been carried out at six temperatures between 240 and 400K and at pressures up to 20MPa. A total of 108 (p,ρ,T) data for the first mixture and 134 for the second one are given. These measurements were performed using a compact single-sinker densimeter based on Archimedes’ buoyancy principle. The overall uncertainty in density ρ is estimated to be (1.5·10 −4 ·ρ+2·10 −3 kg·m −3 ) (coverage factor k=2), the uncertainty in temperature T is estimated to be 0.006K (coverage factor k=2), and the uncertainty in pressure p is estimated to be 1·10 −4 ·p (coverage factor k=2). The equipment has been previously checked with pure nitrogen over the whole temperature and pressure working ranges and experimental results (35 values) are given and a comparison with the reference equation of state for nitrogen is presented.