Efficient separation of magnesite from dolomite remains challenging owing to the similar floatability of the two minerals. Therefore, understanding the adsorption behaviors of surfactants (depressants and collectors) on their surfaces is essential for improving separation efficiency. In this study, the ability of sodium tripolyphosphate (STPP) to separate magnesite from dolomite was evaluated in sodium oleate (NaOl) systems, with an emphasis on its effect on the depression of dolomite. The mechanisms underlying the differential adsorption and depression of STPP were investigated through surface analyses, namely Fourier transform infrared spectroscopy, zeta-potential analysis, adsorption-quantity analysis, and X-ray photoelectron spectroscopy. The flotation results indicated that STPP facilitated the removal of dolomite from magnesite in the NaOl system because it significantly depressed dolomite, whereas its effect on magnesite was negligible. Surface analyses revealed that the preferential adsorption of STPP on dolomite was due to the interaction between the -PO4 tetrahedra of STPP and Ca sites. Furthermore, in the presence of STPP, small amounts of NaOl were adsorbed onto dolomite, inhibiting the flotation of dolomite. However, substantial amounts of NaOl were adsorbed onto magnesite, maintaining the good floatability of magnesite. Therefore, STPP can be considered as a promising depressant for separating magnesite from dolomite via direct flotation. (C) 2020 Elsevier B.V. All rights reserved.