The acid ionization constant of the carboxylic acid group of fluorene-9-carboxylic acid in aqueous solution at 25 °C was determined to be pKa = 3.61, using a method based upon the increase in solubility of this sparingly soluble substance brought about by its ionization. This corrects an earlier considerably different value of this quantity determined by equating pKawith the pH measured at half-neutralization. The hazard inherent in the latter method when applied to sparingly soluble substances is discussed.