Syndiotactic polystyrene (sPS) forms co-crystal structures with a variety of chemical compounds. The guest exchange procedure is an effective way to prepare sPS co-crystals. The substitution is caused by exposing an sPS co-crystal film to a vapor, a liquid or a solution of the new guest molecule. In particular, the addition of an additive, such as chloroform, to the liquid of new guest molecule promotes the guest exchange process and makes bulky molecules easily incorporated into the crystalline region of sPS co-crystals. In order to obtain further information about the influence of additive as accelerant, the sorption and desorption processes of new and old guests during the guest exchange of sPS/chloroform (CHCl3) co-crystal film were followed with ATR FTIR spectroscopy. A series of n-alkanes from n-hexane to n-decane and deuterated chloroform (CDCl3) were employed as new guests and an accelerant additive. The apparent diffusion coefficients of the new and old guests were evaluated from the intensity changes of their IR bands. It has been shown that the addition of CDCl3 promotes the guest exchange between the original guest (CHCl3) and n-alkanes significantly and this effect becomes more pronounced as n-alkane chain-length increases. The diffusion of n-alkanes in atactic polystyrene was also studied for comparison, which suggested that the additive plays an important role at the stage of n-alkane uptake from the amorphous region to the crystallites. (C) 2012 Elsevier Ltd. All rights reserved.