A new technique has been developed for measuring vapour-Liquid equilibria of aqueous solutions containing HCl and HBr in highly diluted vapour phases (< 300 ppm). The analysis of the vapour phase is performed by FT-IR spectroscopy. The reliability and accuracy of the apparatus have been tested with pure water and the wen-known NH3-H2O System. In the presented work binary and ternary aqueous mixtures with the components HCl, HBr and CaCl2 have been investigated. The concentrations of the solutions were adjusted to obtain partial pressures between 0.15 and 30 Pa. The experiments have been carried out at temperatures between 303 and 343 K. Equilibrium data in this range of temperature and concentration are important especially for the design of flue gas cleaning equipment. The measurements of the HCl-H2O system yield lower partial pressures of HCl in comparison with data taken from literature.