Multilevel operation is a topic of much current research in the field of phase-change memory materials, representing the most feasible method for increasing memory density beyond the ultimate scaling limits of the cell size. In this work, we present a combined experimental and ab initio molecular dynamics study of the formation of intermediate states during the crystallisation of Ge 2Sb 2Te 5 (GST). A single intermediate resistance level is formed within a narrow voltage window, and simulations suggest this consists of microscopic crystallites embedded in a bulk amorphous phase. These findings are interpreted within the framework of classical nucleation theory, and a mechanism is proposed to explain the formation of the intermediate state. Our findings suggest that it may be difficult to obtain multiple intermediate states reliably during the crystallisation of Ge 2Sb 2Te 5 and shed light on the fundamental limitations of using this method for multilevel programming.