Measurements on iron containing interstitial nitrogen are discussed. Relaxation phenomena with different relaxation times can be resolved. Unequal initial stiffness of the domain walls gives rise to satellites of the main relaxation. The activation energy of the main relaxation and its satellites coincides with that known for bulk diffusion. Short demagnetization of the relaxed material prior to time decrease results in wall movement during the measurement and in apparent longer relaxation times. The method has been applied for the determination of the activation volume for interstitial diffusion. Preliminary results of measurements on single crystals are given.