Alloys based on TiAl intermetallics are potential candidates for high temperature applications in e.g. aero engines or automotive engines because of their low specific weight and good high temperature strength. To improve their oxidation resistance at temperatures up to 1000°C the halogen effect offers an innovative and cost-effective way. The addition of small amounts of halogens into the surface leads to the preferential formation of gaseous aluminium halides which are oxidised to aluminium oxide during their outward migration forming a dense, protective and slowly growing alumina scale on the surface. In this paper two methods were used to apply halogens to the surface, ion implantation (F and Cl) and a liquid phase process (F). Ion beam analysis with detection limits in the ppm-range was applied to quantify the needed amount of halogens to achieve the halogen effect. Thermocyclic oxidation experiments at 900°C were performed in laboratory air and wet air. Depth concentration profiles of fluorine were measured by PIGE within the first 1.4 μm without destruction of the sample before and after oxidation. Furthermore, the loss of fluorine during heating up and oxidation was measured characterising the stability of the effect. Simultaneous RBS-measurements of the O-, Al- and Ti-depth profiles prove the formation and growth of an almost pure alumina scale. Correlation with the fluorine profiles validates the proposed model for the halogen effect. Furthermore, metallographic methods, REM, EPMA, AES and the proton micro beam (PIXE) were applied to study cross-sections. A virtually pure alumina scale was found after F-treatment and oxidation up to 1500 hours at 900–1000°C in air. The fluorine depth profiles after ion implantation and liquid phase treatment, respectively, show similar levels for both methods before and after oxidation. The development of the fluorine interfacial concentration underneath the oxide scale as a function of oxidation time and temperature was recorded. The results are discussed in the light of the existing model considerations on the halogen effect and with regards to differences in the behaviour between F- and Cl-doping.