The photochemical trans-cis isomerization of graft copolymers of azo dyes onto starch was studied in liquids spectrophotometrically. The relationship between the absorption spectra and spatial hindrance was investigated. The experimental results showed that the structure of the graft copolymers had a marked influence on photoisomerization. The smaller the spatial hindrance was, the easier the photoisomerization was. The copolymers showed better photoisomerization than their corresponding monomers, and this showed that the backbone of starch in the main chain influenced not the maximum-wavelength position of the copolymers but the retroactive structure of the copolymers and favored their photoisomerization. In addition, the thermal cis-trans isomerization rate of the graft copolymers was studied in various polar solvents. The results suggested that the solvent polarity had a large effect on the thermal back-isomerization. (c) 2006 Wiley Periodicals, Inc.