We present the results of an extensive molecular dynamics study of selfinterstitial atom (SIA) clusters containing up to 37 defects over a wide range of temperatures in iron. A long simulation time and high statistics of defect jumps allowed a detailed treatment of the data to be performed. Diffusion exhibits a change in mechanism from three-dimensional to one-dimensional for clusters of 4–7 SIAs. Stable sessile configurations present in the diffusion process are described and their influence on the diffusion parameters is discussed. Diffusion coefficients, correlation factors estimated, and mechanisms observed, are compared with previously published results, and the influence of the interatomic potential is considered.