The structure of very thin iron silicides (d(Fe) < 3 Angstrom) grown by solid phase epitaxy (SPE) on Si(1 1 1) is investigated by means of scanning tunnelling microscopy (STM) and low energy electron diffraction (LEED). A phase transformation, associated with a change in the surface periodicity, from a (2 x 2)-the usual surface reconstruction on metastable FeSi2 layers-toward a c(4 x 8) periodicity is observed as a function of the annealing temperature. This transformation results from a reorganisation of iron vacancies below the topmost Si layer(s). (C) 2003 Elsevier Science B.V. All rights reserved.