Near-critical and supercritical CO2 is used to facilitate the impregnation of additives, vanillin and L-menthol, into cellulose acetate (CA). SCF technology applied at low pressures is a viable approach for this application since CO2 only has to dissolve low molecular weight compounds and not the CA fiber. Hence, impregnation operating temperatures are kept under 60 degrees C and optimum operating pressures are less than 2000 psig and are typically closer to 1250 psig. The optimum operating conditions are intimately linked to the mass transfer and phase behavior characteristics of polymer-CO2, polymer-additive, and additive-CO2 mixtures. It is possible to impregnate up to similar to 10 wt% vanillin or L-menthol into CA fiber as verified by gravimetric, TGA, and TGA/MS analyses. SEM analysis of the CA fiber shows that the fiber does not undergo structural changes during this impregnation process even when the fiber is rapidly depressurized. At atmospheric conditions, the additives are in solid form in the CA fiber and slowly diffuse from the fiber at a rate fixed by solid diffusion from a semi-crystalline polymer matrix. Hence, a significant amount of additive is still present in the fiber after one month. (c) 2008 Elsevier Ltd. All rights reserved.