Generalized equations of the Burnett and Burnett-isochoric methods have been derived for adsorbing gases. A procedure is presented for accurate calculations of the amount-of-substance density and virial coefficients. When adsorption in the apparatus is absent these equations simplify to the well-known forms (Eqs. (1), (2), (4) and (9)) appearing previously in the literature. Data for steam and for a mixture of 0.1017 mole fraction of water in carbon dioxide are used as test cases. These experiments were performed at three temperatures from 448.15 to 498.15 K, at 25 K intervals, and pressures up to 2270 kPa for steam and 9000 kPa for mixture. The Brunauer-Emmett-Teller adsorption model for steam and the Langmuir model for mixture were used to characterized physical adsorption within our Burnett-isochoric apparatus. Densities, virial coefficients, adsorption constants and composition as a function of pressure were calculated.