Curcumin [bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione] was studied by means of UV–VIS absorption spectroscopy and nanosecond laser flash photolysis in 1,4-dioxane–water mixtures in a series of dioxane–water volume ratios. The transient characteristics were found to be dependent on the amount of water. In pure dioxane the triplet state of the molecule in its enolic form was detected (λ_max = 720 nm, τ = 3.2 μs), whereas upon water addition, the diketo form was found to prevail, because of the perturbation of intramolecular H-bonded structure. This led to hydrogen abstraction from dioxane by curcumin triplet state and the formation of the corresponding ketyl radical (λ_max = 490 nm, τ ∼ 10 μs). Laser flash photolysis measurements, carried out in solvents of different polarity and proticity (benzene, cyclohexane and various alcohols), allowed the transient assignments to be confirmed, supporting our interpretation.