Comprehensive (p,rho, 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 400 K and at pressures up to 20 MPa. A total of 108 (p, rho, 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 rho is estimated to be (1.5 . 10(-4) . rho + 2 . 10(-3) kg . m(-3)) (coverage factor k = 2), the uncertainty in temperature T is estimated to be 0.006 K (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. (C) 2005 Elsevier Ltd. All rights reserved.