The photochromic behavior of the spiropyran 1',3'-dihydro-8-methoxy-l',3',3'-trimethyl-6-nitrospiro[2H-1benzopyran-2 ,2'-(2H)-indole] has been investigated in thin films of polycaprolactone-poly(ethylene succinate) blends containing different weight ratios of poly(ethylene succinate) (PESu) and spiropyran. The effects of molecular weight and end group content (-COOH and -OH) of PESu into the coloration-decoloration rate of the fabricated photochromic films have also been examined. The films were exposed to UV radiation (365 nm) at ambient temperature and the absorbance spectra of each polymer blend film were recorded for different time periods. The increase of the amount of PESu in the polymer substrate from 20 to 40 wt % shifts the absorbance towards shorter wavelengths. The same trend was also observed with the increase of carboxyl and hydroxyl groups of the macromolecular chain of the added PESu. Interactions between the polar polymer groups and the spiropyran species, which are formed under the UV excitation, seem to contribute to the color stabilization of the photochromic films. The photochromic response revealed unexceptional repeatability between the slightly colored spiropyran and the intense colored amphoteric species for all the examined polymer matrices. The decoloration rate of the spiropyran-polymer blends follow the kinetic of a first order reaction. (c) 2007 Wiley Periodicals, Inc.