The spectrophotometric technique described in a previous communication has been developed further to study the phase and chemical equilibrium for (sulphur dioxide + water) at elevated temperatures pertinent to flue gas purification. Both the vapour and the liquid phase have been analysed in situ. The analysis of the vapour phase has been performed with the help of a diode array u.v. spectrophotometer equipped with silica fibre light guides. The liquid phase has been analysed using a fibre-optic coupled immersion probe. Experimental results for the equilibrium partial pressures of (sulphur dioxide + water) at very low SO2 concentrations, typical for flue gases, are reported at T = (298.20, 312.95, 322.70, and 332.50) K. The chemical equilibrium in the liquid phase is also presented in terms of dissociated and undissociated sulphur dioxide in aqueous solution.