Some (AlGa)Sb layers grown by molecular beam epitaxy were found unexpectedly to be n type. Characterizing these layers by secondary ion mass spectrometry it was observed that they contained selenium (Se). Systematic analyses showed that selenium was present in all our antimonide layers, even those which were p type, at concentrations between 6×1014 and 3×1017 cm−3. The thermodynamical study of this contaminant incorporation led us to conclude that it comes from the solid antimony used for growths. That was confirmed by spark source mass spectrometry investigations. During growths, selenium behaves as other elements of group VI, sulphur for example, and this behavior is described by a simple kinetic model. It incorporates more easily at low substrate temperatures and saturates at levels depending on the antimony (Sb4) flux. At higher substrate temperatures, the incorporation is balanced by desorption according to an activation energy of 3.2 eV. As a consequence, selenium constitutes an accurate thermal probe to follow substrate temperature variations during growths. As could be expected, sulphur was also found to contaminate (AlGa)Sb films at levels of the order of a few 1015 cm−3. However, oxygen was not detected, probably being lower than 1016 cm−3 the detection limit of the analytical technique. This residual doping of antimonides by chalcogens presumably compensates their natural p‐type doping, but relationships with mobilities have not been evidenced yet.