Solubilities, densities, and refractive indices for the system (NaBO2 + KBO2 + H2O) at T = (298.15 and 323.15) K and P = 0.1 MPa have been experimentally determined by adopting the method of isothermal dissolution for the first time. This system contained two crystallizing fields corresponding to NaBO2 center dot 4H(2)O and KBO2 center dot 4/3H(2)O and one invariant point at 298.15 K, while it contained three crystallization regions corresponding to NaBO2 center dot 4H(2)O, NaBO2 center dot 2H(2)O, and KBO2 center dot 4/3H(2)O and two invariant points existed at 323.15 K. The solution density and refractive index of the system at (298.15 and 323.15) K were similarly increased first and decreased later with the increase of NaBO2 concentration. The calculated results of the refractive index and density using empirical equations were in good agreement with the experimental values of this work.