The Zeta potential of moderately soluble (0.13 M) salts such as borax is difficult to measure due to the high ionic strengths involved under saturated conditions. Measurements reported in the literature are limited to the non-equilibrium technique using conventional Laser-Doppler electrophoresis (LDE) which does not represent the actual conditions. This problem can be overcome with the Phase Analysis Light Scattering technique (PALS.) In this technique, measurement of low mobilities under moderate ionic strengths, even up to 1 M is possible. While non-equilibrium LDE measurements for borax reported in the literature yield an iep of pH 9, the PALS technique indicates that borax is negatively charged in the practical pH region of interest. The PALS technique reveals that the potential determining ions for borax are B4O72- and possibly Na+. The PALS results were used to elucidate the flotation mechanism of borax with anionic collector (sodium dodecylsulfate) and cationic collector (dodecylamine hydrochloride). All these data are used to address the crucial finding that electrostatic interactions still predominate at moderate ionic strengths in borax/collector system.