Hydroxyapatite was found to be a very efficient heterogeneous catalyst for the preparation of chalcone derivatives via Claisen-Schmidt condensation using microwave irradiation. The impact of water on the catalyst reactivity was studied and was thought to act as co-catalyst due to the high activation observed in its presence. To investigate the origin of this activation, different organic solvents of similar or higher microwave absorbance as/to water were also tested, and it was confirmed that water is acting as co-catalyst when combined with hydroxyapatite, making the process highly efficient. The catalyst was easily recovered and efficiently re-used. (c) 2009 Elsevier B.V. All rights reserved.