The measurements of speed of sound for two mixtures of (N2 + CO) are reported. The data have been
obtained using a spherical resonator for two isotherms T = (273.15 and 325.15) K and pressures up to
10 MPa. The uncertainty study is detailed and the total uncertainty of the speed of sound is not worse
than 0.016%. The results have been compared with the GERG-2008 equation of state