Cadmium substituted strontium hydroxy- and fluoro-apatites, Sr(10-x)Cdx(PO4)(6)Y-2 (Y = OH and F), have been prepared as single phases in the 0 <= x <= 4 interval for the former and 0 <= x <= 6 for the latter compound, respectively. The refinements of the X-ray full powder patterns allowed the structure determination of nine samples, and showed a preference of cadmium atoms for the M(1) site in fluoroapatite samples. IR investigation gave information about the nature of the metal oxygen interactions. The obtained results are used for a comparative discussion about the factors which drive the mutual substitution of Ca, Sr, Cd and Pb in apatites. The important role of electronegativity and polarisability, as well as of ionic radii, is evidenced. (C) 2008 Elsevier Ltd. All rights reserved.