The Prussian blue analogue K 0.4 Co 1.3 [Fe(CN) 6 ]·nH 2 O has previously been shown to exhibit three distinct phases at low temperature. In addition to the well-studied high-temperature ‘high spin’ and low-temperature ‘low spin’ phases, with unit cells of ∼10.3 and ∼9.95Å, respectively, an intermediate phase with unit cell a=10.09Å was recently reported, although it existed as a minority constituent in a mixed-phase material. Herein we report the synthesis of 130nm K 0.4 Co 1.3 [Fe(CN) 6 ]·4.4H 2 O particles that form solely the intermediate phase at temperatures below the normal charge transfer induced spin transition. Compared with those formed from a bulk synthesis, the nanosized particles are shown to have the same chemical composition at high temperature, but they undergo different structural transitions with thermal cycling.