In the associated silicate rocks at Oldoinyo Lengai concentrations of all three halogens increase from primitive olivine melilitite to evolved combeite-wollastonite nephelinite, demonstrating their incompatible behavior during magmatic differentiation. Relatively constant Cl/Br ratios, but strongly decreasing F/Cl ratios, in these samples are explained by moderately incompatible behavior for F compared to pronounced and very similar incompatibilities for Cl and Br. Our data further imply that during silicate-natrocarbonatite melt separation all three halogens show a strong affinity for the natrocarbonatite. F and Cl are equally enriched in the natrocarbonatite while Cl and Br are slightly fractionated from each other with a preferential partitioning of Cl relative to Br into the natrocabonatite melt.