Hydroxyapatite (HAP) particles with different morphologies were precipitated from homogeneous calcium/citrate/phosphate solutions at physiological temperature. Small variations of the starting solution pH in the range 7.4 < pH < 8.5 made it possible to switch the precipitated particle morphology from a micrometric bundlelike to a nanometric needlelike shape. The role of the existing citrate species as calcium chelates is here discussed within the framework of particle nucleation and growth mechanisms. While temperature-dependent calcium citrate complex (Cacit) stability is here suggested to control the free calcium availability and thereby the nucleation rate, the adsorbed citrate species are proposed to control the nanoparticle stability. Moreover, an attempt to detail the role of citrate species in the ordered aggregation of HAP nuclei leading to the observed peanut and bundlelike microparticles morphology is also presented. (c) 2007 Elsevier Inc. All rights reserved.