The effect of molecular weight of poly(epsilon-caprolactone) (PCL) on the formation and stability of inclusion complexes (ICs) between alpha-cyclodextrin (alpha-CD) and PCL was investigated by FTIR, WAXD, and DSC measurements. ICs between alpha-CD and PCLs with a wide range of number-average molecular weight, M-n = 1.21 X 10(4) - 1.79 x 10(5), were prepared by mixing the aqueous solution of CD and acetone solution of PCL followed by stirring at 60 degrees C for 1h and at the room temperature for 1 day. FTIR, WAXD, and DSC measurement showed the PCL chains were included into the alpha-CD cavity, and the crystallization of PCL was suppressed in the alpha-CD cavity. Stoichiometry and yield of each IC varied with the molecular weight of guest PCL, and the effect of IC formation on the crystallization behaviour of guest polymer decreased with the increase of molecular weight of guest polymer. (c) 2005 Wiley Periodicals, Inc.