We have investigated the effect of pressure on excited-state intramolecular proton transfer (ESIPT) in 3-hydroxyflavone dissolved in four polymeric matrixes-poly(methyl methacrylate) (PMMA), poly(vinyl acetate) (PVAc), poly(acrylic acid) (PAA), and cellulose acetate (CAC). We have also determined the change in normalized equilibrium constant (K*) and the change in partial molar volume (Delta(V) over bar) as a function of pressure. In all matrixes, with an increase of pressure, the fluorescence intensity of the phototautomer decreases. In PMMA, PVAC, and CAC, the equilibrium constant (K) and change of partial molar volume (Delta(V) over bar) are sensitive to pressure below 30 kbar. The results imply that ESIPT of 3-hydroxyflavone is effectively suppressed by pressure. The anomalous proton-transfer behavior of 3-hydroxyflavone in PAA can be explained by protonation of the carbonyl oxygen.