Objective: Hydrochlorothiazide is a drug that lowers blood pressure by decreasing Na+ and Cllevels while increasing the rate and volume of urine excretion. This drug is used alone or in combination with other antihypertensive medications. However, the action mechanism of such an important drug is unknown, and in the literature, there is only one study on the value of the ionization constant of the compound in hydroorganic systems and no study on the value of the ionization constant of the compound in water. Therefore, in this study, an attempt was made to determine the ionization constant of hydrochlorothiazide in various binary acetonitrile-water mixtures and to determine the ionization constant of hydrochlorothiazide in water by using these values through various extrapolation methods. Material and Method: The ionization constant of hydrochlorothiazide was determined in a binary mixture of 11%, 13%, and 15% (v/v) acetonitrile and water at 30oC, a flow rate of 1 ml/min, and an X Terra C18 column. To determine the ionization constant of hydrochlorothiazide, the pH retention relationship was evaluated using the linear solvation energy relationship (LSER) model. To determine the ionization constant values in water, the data obtained from the LSER model were evaluated using the Yasuda-Shedlowsky method and the mole fraction- sspKa method. Result and Discussion: The agreement between the thermodynamic wwpKa1 values calculated by the two methods is remarkable and consistent with those found in the literature for the organic solvent-water environment. Considering this information, the accuracy and reproducibility of the determined wwpKa1value of hydrochlorothiazide becomes clear. Moreover, the determined values are the first thermodynamic wwpKa1data in the literature obtained by any analytical method for hydrochlorothiazide.