A new experimental technique based on spectrophotometric measurements is presented for the study of vapour-liquid equilibrium for (sulphur dioxide + water). The in situ analysis of the vapour phase is performed with the help of a diode array UV-Vis. spectrophotometer equipped with silica fibre light guides. Experimental results for the equilibrium partial pressures of (sulphur dioxide + water) at low SO2 concentrations are compared with existing literature values. The liquid phase has been analysed independently using ion chromatography and confirms the vapour phase measurements. The results favour a value of 0.0139 at T = 298.15 K for the equilibrium constant for the first dissociation reaction of hydrated SO2. In addition to the in situ analysis of the gas phase, a method is established for the spectrophotometric analysis of the dissolved molecular sulphur dioxide.