The effect of sulfur dioxide partial pressure and reaction temperature on the composition of the poly-hydriodic acid solution produced in the Bunsen reaction, is an unit reaction in the IS process for thermochemical hydrogen production, was studied experimentally under iodine saturation. The ratio of (HI/(HI + H(2)O)) of the polyhydriodic acid solution assuming an ideal desulfurization was increased with sulfur dioxide partial pressure and reaction temperature. An experimental equation for the ratio of (HI/(HI + H(2)O)) was obtained as a function of sulfur dioxide partial pressure and reaction temperature.