Due to the high chemical reactivity of GaSb surfaces, many commonly used aqueous sulfide passivation techniques lead to the growth of surface oxides that degrade device performance. We have developed a non-aqueous passivation regime consisting of Na2S/benzene/15-crown-5/oxidant. The use of a non-polar, aprotic organic medium required the addition of a specific chelating agent, i.e. a 15-crown-5 ether, to solubilize sodium sulfide, and organic oxidizing agents, such as anthraquinone and benzophenone, to act as electron acceptors. The surface optical and chemical properties of GaSb surfaces after aqueous and non-aqueous sulfide treatments were compared. Non-aqueous passivation resulted in higher PL intensity, lower oxide content, and a less amount of elemental Sb than aqueous passivation.