In this study, sorption and diffusion of carbon dioxide (CO2) in wood-fiber/ polystyrene composites were investigated. The effects of gas pressure and fiber content on the solubility and diffusion coefficients were evaluated. A statistical analysis indicated that pressure is more important than fiber content in determining the solubility and diffusivity of CO2. An increase in saturation pressure causes an increase in the solubility and diffusion coefficients, whereas inclusion of the fibers decreases both of these properties. Models were developed to predict the uptake and diffusion coefficients of CO2 in the composite samples as functions of pressure and fiber content. A theoretical model based on Henry's law and the Langmuir equation compared favorably to the experimental data for CO2 solubility. This dual mode model also described both the transient sorption and desorption data, but only if the concentration-dependent value of diffusivity was treated as a history-dependent parameter.