Anelastic effects caused by carbon and vacancy diffusion in Fe3Al based alloys with and without strong carbide forming elements (Ti and Nb) are investigated by internal friction measurements. The decrease of the Snoek peak in Fe–26Al–2Ti and Fe–26Al–0.3Nb alloys with respect to the Fe–26Al alloy is related to a decrease in the amount of interstitially dissolved carbon (C). The so-called X peak, which is observed in the Fe–26Al alloy, also almost disappears after alloying. In order to elucidate the vacancy contribution to the origin of the X peak radiotracer measurements of 59Fe diffusion were performed. The results indicate that the change in the interstitially dissolved C concentration is the main reason of the observed changes in the X peak, although a certain influence of Ti and Nb alloying on the total vacancy concentration may be deduced from the diffusion study.