The primary goal of this paper is to investigate the separation performance of bicarbonate salts through nanofiltration (NF) membrane LES 90. The experiment results indicate that the observed transmissions of total carbon (TC) at pH 7 are much higher than that at pH 8.3. This phenomenon can be explained by the dynamical equilibrium of weak electrolyte. The observed transmissions of both HCO3- and H2O center dot CO2 are calculated. The regulation of observed transmissions of HCO3- with flux and concentration is similar to the single-charged strong electrolyte anions, while the observed transmissions of H2O center dot CO2 keep almost unchanged at an average value of 0.81. Finally the fraction summation model proposed in Our previous paper is applied, resulting in good agreement with the experiment results.