Abstract: Computer simulation of Purex process can facilitate the optimization and improvement of the process. Nitrous acid is inevitably present in Purex process, which also affects the extraction of some important nuclides, thus, it is desired to build a HNO₂ distribution model which could be incorporated into Purex computer simulation code. In this work base titration and diazotization-coupling reaction was used to determine the concentration of HNO₃ and HNO₂ in both equilibrium aqueous and organic phase, to determine their distribution ratio. Nitrous acid concentration had little effect on the distribution ratio of HNO₃ and HNO₂. As HNO₃ concentration increases, the distribution ratio of HNO₃ increases first then decreases, while the distribution ratio of HNO₂ decreases. The distribution ratio of HNO₃ and HNO₂ increase with TBP concentration. A formula was established to correlate the distribution ratio of HNO₂ with HNO₃ concentration and free TBP concentration. The calculated results agree well with the experiment results.