The effect of pressure on excited-state intramolecular proton transfer (ESIPT) is studied in three polyhydroxyflavones: 3,7-dihydroxyflavone, 3,5,7-trihydroxyflavone (galangin), and 3,3',4',7-tetrahydroxyflavone (fisetin), dissolved in poly(methyl methacrylate) (PMMA), poly(vinyl acetate) (PVAc), poly(acrylic acid) (PAA), and cellulose acetate (CAC). The unusual photophysical properties of polyhydroxyflavone in PAA are interpreted in terms of protonation of the carbonyl oxygen. We calculate the ratio of the equilibrium constant at elevated pressure to that at 1 atm, K (p)/K (0), and the difference of partial molar volume between the normal state and the phototautomer, DeltaV. The results indicate that pressure effectively suppresses intramolecular proton transfer in polyhydroxyflavone. K (p)/K (0) and DeltaV show a strong pressure dependence in the range of 10-30 kbar.