The diffusion coefficients and solubility of CO2 in methyl cellulose (M-w = 40 000 g.mol(-1)) and sodium carboxymethyl cellulose (CMC) of different molecular weights, M-w = (90 000, 250 000, and 700 000) g.mol(-1), were determined by a gravimetric method. The MBS (magnetic suspension balance) was used for measurements at three different temperatures, (313, 333, and 353) K, and pressures up to 30 MPa. High values of solubility (up to 36.11 mol %) and diffusion coefficients [(2.55.10(-9) to 8.61.10(-8)) cm(2).s(-1)] were obtained. The solubility of CO2 in the polymers depends on the temperature and pressure, while diffusion coefficients are concentration-dependent. The solubility and diffusivity in the various CMCs are influenced by molecular weight and the degree of substitution.