The formation of isolated circular islands submitted to misfit strain has been theoretically investigated on the surface of a thin wetting layer of constant thickness deposited on a substrate by calculating the chemical potential of the island atoms. It is found that the island size selected by the misfit strain depends on the difference between the shear moduli of the islands (and wetting layer) and the substrate. An analytical expression for the island radius is derived from the first order development in the shear modulus difference. (C) 2013 Elsevier B.V. All rights reserved.