Decomposition of solid solution taking into account the grain boundaries segregations is considered in the framework of generalized Cahn-Hilliard model for finite systems. Two types-"stripe" and "drop-wise"-of the surface-directed decomposition found below spinodal depend on concentration of a wetting component. We demonstrate that grain boundaries segregations are able to drastically change the phase equilibrium inside the grain when its size becomes less than the critical value. As a result, decomposition of the solid solutions easily appears for nanograined materials. (C) 2004 Kluwer Academic Publishers.