The development of the current reference equation of state for natural gas and other related mixtures (GERG 2008) was based on a wide up-to-date database of experimental thermodynamic properties. This extensive data bank showed, however, a significant lack of data for many of the possible binary mixtures of natural gas components. In fact, density data for carbon monoxide-nitrogen mixtures were only available for one single composition (0.03 CO + 0.97 N2) and in a limited temperature range, (273 to 353) K In this paper density data are presented for two new binary mixtures, (0.05 CO + 0.95 N2) and (0.10 CO + 0.90 N2), along seven isotherms between (250 and 400) K and pressures up to 20 MPa. These data allow the predictive capacity of the GERG model to be tested and, if necessary, to contribute to an improvement in its mixture parameters. The measurements were carried out using an accurate single sinker densimeter with magnetic suspension coupling to achieve the highest accuracy. Results showed very good agreement with the GERG model, since relative deviations in density between experimental and estimated values are within a 0.05 % band. Interaction second virial coefficients were fitted and reported.