In industrial crystallization, a successful concentration measurement technique helps controlling the supersaturation, and thus obtaining the desired crystal size distribution will be possible. But for a two-component solution in equilibrium, a rapid concentration determination could be difficult. During the crystallization of sodium hydrogenocarbonate (sodium bicarbonate), in solution, there is an equilibrium between NaHCO3, Na2CO3 and CO2. In this study, a supersaturation determination method has been devised and developed for the sodium carbonate-bicarbonate system. The objective is to relate the density variation of the solution to the supersaturation of NaHCO3, so that the evolution of the supersaturation can be followed during the crystallization process. The method is to measure the density variation between the original solution and the solution after a desaturation or a saturation step, while minimizing the Na2CO3 influence. The desaturation and saturation steps are carried out in a glass tube filled with NaHCO3 crystals. This supersaturation determination method has been successfully applied, and the validity of the method was confirmed on NaHCO3 saturated solutions, where the Na2CO3 is controlled by CO2 partial pressure. This methodology could be interesting not only for the study of the crystallization in two-component systems but also for solutions containing impurities. (C) 2003 Elsevier B.V. All rights reserved.