The phototransformation of benzimidazole (BZ) and of the benzimidazole pesticide thiabendazole (TBZ) was investigated in aqueous solution in the absence and presence of the supramolecular host cucurbit[8]uril (CB8). ESI-MS and NMR reveal that both compounds form stable 1:2 host-guest complexes with CB8 (BZ2@CB8, TBZ2@CB8). The phototransformation of free BZ leads to dehydrodimerization, while for TBZ the photoreactivity leads to BZ, benzimidazole-2-carboximide and 2-acetylbenzimidazole. Inside CB8, BZ undergoes photohydrolysis to form 2-aminoformanilide, while for TBZ2@CB8 additional photoproducts were observed which are pH dependent. At pH 1.2 photolysis of TBZ2@CB8 leads to new red-shifted photoproducts with extended π conjugation. © The Royal Society of Chemistry and Owner Societies.