In this work, MBE growth of InAs islands on InAlAs layers lattice mismatchcd to GaAs substrates has been studied. Both pseudomorphic and metamorphic InAlAs buffer layers were used as a template prior to the deposition of InAs. The effects of incorporating aluminium atoms in the buffer layer and the strain relaxation in the InAlAs layer os well as the InAs coverage on the density and the size of the islands are shown.