The preparation of GaSb(100) surfaces by ultraviolet (UV) irradiation, prior to molecular beam epitaxy (MBE) growth, has been investigated. After optimization of the UV irradiation parameters (type of lamp, distance between the lamp and the sample, time exposure), GaSb surfaces present no detectable carbon contamination, a stoichiometry close to the MBE surface one, and a very good morphology. The oxide layers grown an the substrate surface have been analyzed by Auger electron spectroscopy and x-ray photoelectron spectroscopy. Large differences in the oxide composition, resulting from variations of the UV irradiation conditions, have been observed and analyzed. We discuss the experimental results and particularly the carbon desorption, in terms of oxide growth mechanisms and we propose a model of oxide growing in the optimal conditions.