A new experimental technique based on spectrophotometric measurements is presented for the study of phase and chemical equilibria of aqueous solutions containing volatile weak electrolytes, e.g., SO2, NO2. The newly developed apparatus allows the in situ analysis of both the vapour and the liquid phases. The in situ analysis of the vapour phase is performed with the help of a diode array W-vis spectrophotometer equipped with silica fibre light guides. The liquid phase has been analysed using a fibre optic-coupled immersion probe. The technique is most suitable for the VLE measurements at elevated temperatures, The new technique has been validated by measuring the phase and chemical equilibria of SO2 + water system at 298, 313, 323 and 333 K and comparing these with the best available literature values. The measurements have been performed at low SO2 concentrations, which are typical for the flue gases. The studies have been extended to multicomponent systems by introducing CaCl2 to SO2 + water system. The measured phase and chemical equilibria for this ternary system is discussed in terms of known thermodynamic models.