Styrene and acrylic acid were impregnated into a series of polyamide products (nylon1212, nylon1010, nylon66, nylon6) using supercritical CO2 as the additive-carrier and substrate-swelling agent. The impregnation efficiency of low molecular weight additives into substrates is attributed to complicated interactions among the system: (1) loading of additives in substrates, (2) dissolving of additives in CO2 phase, (3) swelling and plasticizing of substrates by CO2 For the first time solubility parameter was introduced to discuss the impregnation efficiency. It was found that the relative solubility of additive in the polymer substrate and CO2 is a major factor governing the incorporated amount; yet swelling of the substrate and CO2-induced crystallization also contribute to the value. The study generalizes complex factors influencing the impregnation possibility of different systems. (C) 2004 Wiley Periodicals, Inc.