This paper aims to extend the application field of the hygrometric method to systems containing volatile electrolytes. In this way, the aqueous hydrochloric acid-water solution and the aqueous hydrochloric acid-potassium chloride-water system have been studied with the hygrometric method at the temperature 298.15 K. To take into account the hydrochloric acid volatility, we use a corrected ratio of droplet growth K-Adjusted = 0.0105 + 0.998.K-Exp (K-Exp is the measured ratio of droplet growth) obtained from experimental data of the hydrochloric acid-water system. The water activities and the corresponding osmotic coefficients are deduced from measurements of relative humidities of the HCl-KCl-H2O system against total molality ranging from 0.3 to 7.5 mol. kg(-1) and for different ionic-strength fractions y of HCl y = I-HCl/(I-KCl + I-HCl)) of 1/3, 1/2, 2/3, 3/4, 10/11, and 15/16 at 298.15 K. The results obtained were correlated using the PSC model to predict the solute activity coefficients in a mixture for different compositions of the aqueous solution of the hydrochloric acid system. The obtained results are used to determine the solubility and excess Gibbs energy in the studied molality range.