The equilibrium solubility of Mg-containing beta-tricalcium phosphate (betaMgTCP) with various magnesium contents was determined by immersing betaMgTCP powder for 27 months in a CH(3)COOH-CH(3)COONa buffer solution at 25 degrees C under a nitrogen gas atmosphere. The negative logarithm of the solubility product (pK(sp)) of betaMgTCP was expressed as pK(sp)=28.87432+1.40348C-0.3163C(2)+0.04218C(3)-0.00275C(4)+0.0000681659C(5), where C is the magnesium content in betaMgTCP (mol.%). The solubility of betaMgTCP decreased with increasing magnesium content owing to the increased structural stability and possible formation of a whitlockite-type phase on the surface. As a result, betaMgTCP with 10.1 mol.% magnesium had a lower solubility than that of hydroxyapatite below pH 6.0. betaMgTCP was found to be more soluble than zinc-containing beta-tricalcium phosphate given the same molar content of zinc or magnesium. The solubility of betaMgTCP and release rate of magnesium from betaMgTCP can be controlled by adjusting the Mg content by selecting the appropriate pK(sp).