The composition of the mixtures under study is shown in table 2 into detail. Those mixtures were prepared by the Spanish Institute of Metrology(CEM) bya geometrical process. Two isotherms were measured at T = (273.160 and 325.000) K in pressure range from 0.1 MPa to 20 MPa for two different mixtures. The cylinders in which these gas mixtures were supplied were stored horizontally and rolled frequently to promote mixing.The mixtures were studied