The composition of biogas may vary significantly due to the diversity of production sources, making it essential to have a detailed knowledge of their thermophysical properties in order to develop and validate methods for the estimation of density, heat capacity and calorific value of biogas and biomethane. In this work the thermodynamic behavior of a synthetic biogas-like mixture, composed of methane (50%), carbon dioxide (35%), nitrogen (10%) and carbon monoxide (5%), is studied through accurate (p, rho, T) experimental data obtained by using a single sinker densimeter with magnetic suspension coupling. The mixture was prepared by the gravimetric method at the Spanish National Metrology Institute (Centro Espanol de Metrologia, CEM) and the accurate density measurements have been performed in the temperature range from (275 to 400) K and pressures up to 20 MPa. This work is part of the research project 'Metrology for Biogas' supported by the European Metrology Research Program. Experimental data are compared with the densities calculated with the GERG-2008 equation of state. The deviation between experimental and estimated densities is within a +/- 0.2% band at all temperatures, except at the lower temperature, 275 K, and pressures from (6 to 15) MPa, which shows a higher deviation. (C) 2015 Elsevier Ltd. All rights reserved.