Sorption of CO2 in both the glassy and the rubbery state of an amorphous polyethylenelike polymer was investigated using molecular dynamics simulations. The temperature was chosen such that the system was in its glassy state at low solute concentrations and its rubbery state at large solute concentrations. Both the pressure and the volume isotherms changed character at the transition concentration. The physical origin of these changes was clarified by investigation of the excess thermodynamic properties of the solute both below and above the transition concentration. Dynamical changes occuring at the glass transition were studied through the self-intermediate scattering function of the polymer atoms. This function was found to excellently reveal the difference between the dynamics of the glassy and rubbery state and therefore served as an independent tool monitoring the glass transition.