A certainty/uncertainty study of dual-mode model parameters for gas sorption in glassy polymers is discussed based on a statistical analysis employing about 250 types of glassy polymers from extensive literature data. The dual-mode sorption parameters correlate to various factors, such as the glass transition temperature, fractional free volume, gas permeability, gas diffusivity, and gas solubility. However, a wide variation of the dual-mode parameters is observed for each correlation. Better correlations, such as those involving the Langmuir-site contribution to the total solubility at infinite-dilution (S-H0/S-0), or non-equilibrium excess free volume fraction ratios ((C'(H) /p*)/FFV), appear only among families H of similar glassy polymers. Hence, the dual-mode analysis for gas sorption in glassy polymers can be used as an effective strategy in terms of relative evaluations. However, it is difficult to quantitatively compare the parameter values between different types of glassy polymers. (c) 2005 Elsevier B.V. All rights reserved.