A procedure to obtain monodisperse nanoparticles of cobalt(1) hexacyanoferrate(III) was investigated. To prevent a reaction with [Fe(CN)(6)](3-), Co2+ was stored in the form of a citrate complex at pH 7.8. By lowering the pH to 3.4 by the addition of acid, part of the Co2+ was released and reacted with [Fe(CN)(6)](3-) to form nuclei, followed by their growth via mass-transfer of Co2+ from the citrate complex to the solid. Here, gelatin was employed as a protective colloid. The particles obtained were cubic and fairly monodisperse. The mean particle size was typically 160 nm and varied from 30 to 450 nm, depending on the reactant contents. The formation mechanism associated with equilibrium shifts was supported by a calculated estimation of free Co2+ on the basis of the equilibrium relationships involved in the preparation system.