Multinuclear NMR spectroscopy has been used to monitor synthesis of hydroxyapatite (HAp) from diethyl hydrogen phosphonate and calcium diethoxide in solution at two different temperatures. Acetyl 2-hydroxyethyl phosphonate, bis(2-hydroxyethyl) phosphonate, and acetyl ethyl phosphonate have been identified for the first time in this reaction solution as intermediates. The formation of these compounds is shown to be crucial in controlling the phase purity of the final hydroxyapatite product. A possible mechanism for the formation of acetyl 2-hydroxyethyl phosphonate is discussed.