Using molecular recognition of a-cyclodextrin (alpha-CD), we have successfully constructed a photoresponsive hydrogel system by combination of simple components, a-CD, dodecyl (C-12)-modified poly(acrylic acid) (p(AA/C-12)), and a photoresponsive competitive guest, 4,4'-azodibenzoic acid (ADA). An aqueous solution of p(AA/C-12) exhibited a gel-like behavior because polymer chains form a network structure via hydrophobic associations Of C-12 side chains. When alpha-CD was added to the gel-like aqueous solution, the gel was converted to a sol mixture because hydrophobic interactions Of C-12 side chains were dissociated by the formation of inclusion complexes of alpha-CD with C-12 side chains. Upon addition of ADA to a binary sol mixture of p(AA/C-12) and alpha-CD, alpha-CD interacted predominantly with ADA, and then hydrophobic associations of C-12 side chains were restored, resulting in a sol-to-gel transition. When a ternary gel mixture of P(AA/C-12), alpha-CD, and ADA was irradiated with UV light, ADA was isomerized from trans to cis, and the mixture underwent a gel-to-sol transition because alpha-CD formed inclusion complexes more favorably with C-12 side chains than with cis-ADA. When the ternary sol mixture was irradiated with visible light, ADA was isomerized from cis to trans and the mixture underwent a sol-to-gel transition. Furthermore, these gel-to-sol and sol-to-gel transitions occurred repeatedly by repetitive irradiations of UV and visible light.