In this work we established three from altogether six proton dissociation constants (pK(a)) of hydroxyl groups of hypericin in its monomeric form. The monomeric state of hypericin (5.0 x 10(-6) mol.L-1) in aqueous solution was stabilised by the presence of hydrotropic drug Cromolyn disodium salt (6.0 x 10(-2) mol.L-1). Data show that one acid-base transition occurs with the pK(a) of 7.8 and the other two are characterised by the apparent single pK(a) of 11.5. The spectral changes of hypericin above pH 13 indicate that the last two hydroxyls are deporotonized at this high pH values. (C) 2017 Elsevier B.V. All rights reserved.