The densities of six aqueous ternary systems that contain boric acid or disodium tetraborate and different electrolytes (NaCl, Na2SO4, or K2SO4) were measured at 20, 25, and 30 degrees C. The experimental study of these systems were conducted for four total molality values of the mixtures: m(T) approximate to 0.25, 0.50, 0.75, and 1.0 mol/kg for systems that contain B(OH)(3) and mT approximate to 0.10, 0.12, 0.14, and 0.17 mol/kg for systems that contain NaB4O7. The total molality is calculated as the sum of the boron compounds and the electrolyte molalities. For each total molality, the values of the molar fraction of the B(OH)(3) or NaB4O7 (free of water) were y(1) approximate to 0.25, 0.50, and 0.75. The parameters of the Pitzer model were fit to the experimental density values. The regression analysis was simplified by considering the boric acid or disodium tetraborate to behave as undissociated neutral species. However, it was considered that the second virial coefficients of the neutral species (Inn), were dependent on their molality. Also, using the Pitzer model, we determined the volume of mixing (Delta V-M), at constant total molality of the mixture (m(T)) and at 25 degrees C, for each of the six ternary systems that have been studied.