Particle-size dependence of alloy phase formation in isolated particles has been studied by transmission electron microscopy at room temperature, using alloy particles in the In-Sn binary system. In approximately 16-nm-sized particles, essentially the same alloy phases as those in bulk materials formed. Namely, at compositions near the eutectic composition, a mixture of two crystalline phases formed. However, when the size of particles was decreased down to approximately 10 rim in diameter, a liquid phase, which is not an equilibrium phase in bulk materials at room temperature, formed. The formation of a liquid phase is ascribed to the large suppression of the eutectic temperature associated with size reduction. (C) 2003 American Vacuum Society.