The effect of alpha-cyclodextrin (alpha-CD) on the trans-cis photoisomerization of an azobenzene functionalized polymer (AZO-HPMC) in aqueous solutions has been systematically studied. It has been shown that the cis fraction at the photostationary states and the photoisomerization rate constant increases with increasing alpha-CD concentration, indicating that the trans-cis photoisomerization of AZO-HPMC is accelerated by addition of alpha-CD. This behavior is interpreted in terms of the aggregation between the azobenzene groups. The formation of inclusion complexes between the trans isomers and alpha-CD inhibits the aggregation and consequently increases the mobility and free volume for the isomerization.