Olivine compounds have emerged as important and enabling positive electrode materials for high-power, safe, long-life lithium rechargeable batteries. In this work, the miscibility gap in undoped Li1-xFePO4 is shown to contract systematically with decreasing particle size in the nanoscale regime and with increasing temperature at a constant particle size. These effects suggest that the miscibility gap completely disappears below a critical size. In the size-dependent regime, the kinetic response of nanoscale olivines should deviate from the simple size-scaling implicit in Fickian diffusion. (c) 2007 The Electrochemical Society.