Résumé : Using a 2.1 GeV/c K - exposure in a heavy liquid bubble chamber, the masses of the Ξ hyperons have been measured to be M(Ξ 0) = 1315.2 ± 0.9 MeV/c 2 and M(Ξ -) = 1321.12± 0.41 MeV/c 2. Their mass difference is compared to theoretical predictions. © 1972.