Protocatechuic acid has mixed effects on normal and cancer cells in vitro and in vivo studies[1],it possesses antioxidant,anti-inflammatory as well as antihyperglycemic and neuroprotective activities[2].The enthalpies dilution and dissociation of protocatechuic acid in aqueous potassium chloride or sodium chloride solutions have been measured simultaneously by using mixing-flow microcalorimeter at 298.15 K.The enthalpic interaction coefficients(h2,h3,h4),along with dissociation constant(Ka)and dissociation enthalpy(ΔdisHm),in the range of the salt molality(0 to 0.6)mol·kg-1,were calculated through the fit of the experimental data.The dissociation constant and enthalpy determined by the conductivity method and ultraviolet absorption spectrometry method verified the reliability of the fitted thermodynamic parameters.The fitted results indicated that the enthalpic pairwise interaction coefficients h2 of protocatechuic acid in both aqueous salt solutions are all negative and decrease with the increase of the salt molality.The value of h2 in aqueous potassium chloride solutions is larger than that in aqueous sodium chloride solutions of the same molality.