The purpose of this paper is to study the surface energetics of the polymer excipient cellulose acetate propionate (CAP) in the solid form. The net retention volumes, V (N), for n-alkanes and polar solutes have been measured in the temperature range 353.15-403.15 K by inverse gas chromatography. The dispersive surface free energy, , and Lewis acid-base parameters and , have been determined using V (N) values. The values are decreased linearly with increase of temperature. The value at 353.15 K is 24.50 +/- A 1.54 mJ/m(2), and the temperature gradient was found to be -0.287 mJ/m(2)/K-1. The and values are 0.410 +/- A 0.021 and 1.708 +/- A 0.388, respectively, which suggest that the CAP solid surface contain relatively more basic sites. The K (a) and K (b) values of CAP are compared with the similar values obtained on the cellulose acetate butyrate solid surface.