To study air diffusion in polar firns, and thus evaluate the mean age of air constituents as a function of depth, it is necessary to know the effective diffusivity profile for the studied gas and firn. In this work we compare effective diffusivity profiles obtained by two methods: first, by experimental measurements on firn samples, second, using an inverse gas diffusion model. Results are obtained for three gases, CO2, CH4 and SF6, in firns exposed to different climatic conditions. While measurements give similar results for the different sites, the inverse model shows variations depending on the climatic conditions. We therefore conclude that diffusivities measured on small firn samples cannot be directly used to determine the tortuosity profile of real firn and that diffusion models have to be used. ¿ 2000 American Geophysical Union