A stable hydrogen production via the IS process is relatively difficult because of multiple components and corrosive solution. For stable operations of the IS process, to regulate the composition of Bunsen reaction solution is one of key techniques. For this, a new technique was developed to estimate compositions of Bunsen reaction solution in liquid-liquid phase separation, H2SO4 phase and HI phase, which was targeted on the two solutions in phase equilibrium. In this technique, just two densities measured in the two solution are employed to estimate compositions of two solutions consisting of HI, H2SO4, I2, H2O. The fundamental equations underlying this estimation is relations between densities and compositions. To formulate the relations, densities of simulated Bunsen reaction solutions were measured and regression expressions were formulated. Simulated solution for H2SO4 phase and HI phase in liquid-liquid phase separation were prepared by mixing the reagents to be appropriate composition, their densities were measured by a desktop-type densitometer. Moreover, relational equations for both phases were set up from measured values by the regression procedure. The equations are able to derive the density of optional compositions below 1% of accuracy. Through estimations of both compositions from both densities, averaged differences between estimated and analyzed compositions were below 1.5 mol%. Because of easily and simply execution, this technique is useful for practical use, especially for plant operation and equipment design.