The solubilities in the MCl–M2B4O7–H2O (M = Li, Na) systems at 323.15 K were studied with the isothermal solid–liquid equilibrium method. Both the phase diagrams in the above systems belong to the simple eutectic type. Combining the Pitzer binary parameters of LiCl and NaCl obtained from the literatures, the lacking Pitzer binary parameters of Li2B4O5(OH)4 and Na2B4O5(OH)4, Pitzer mixing parameters θCl–,B4O5(OH)4 2– , ψLi+,Cl–,B4O5(OH)4 2– , and ψNa+,Cl–,B4O5(OH)4 2– , and the solubility product constants of Li2B4O5(OH)4·H2O and Na2B4O5(OH)4·8H2O at 298.15 and 323.15 K were fitted with the experimental solubility data by the Pitzer model. The agreement between the experimental water activities and calculated results with parameters fitted with solubilities in the system LiCl–Li2B4O7–H2O at 298.15 K shows that the parameters fitted with solubilities are reliable. The comparison between the experimental and calculated solubilities of the two systems at 298.15 and 323.15 K shows that calculated solubility data are in agreement with 0.03 of experimental results in the two systems.