Objective: Previous studies (Yan et al., 2001 AADR Abstract #1806) have shown that the stoichiometry of the surface complex governing the metastable equilibrium solubility (MES) of carbonated apatite (CAP) is well-approximated by that of hydroxyapatite for CAP samples of high crystallinity; however, for CAPs having poor crystallinity, there was an apparent decrease in the calcium-to-phosphate (Ca/P) ratio of the surface complex with increasing crystallite microstrain.  The purpose of the present study was to examine in more detail this decrease in the Ca/P ratio of the MES governing surface complex for a CAP sample of low crystallinity.  Methods: A CAP of low crystallinity (~ 6.4 wt% carbonate, crystallite microstrain parameter = 0.559) was prepared by precipitation/digestion at 50 ºC.  MES profiles were determined in a series of 0.1 M acetate buffers with ionic strength = 0.5 at different slurry densities. The pH and strontium/calcium ratios of the dissolution media were varied over wide ranges.  All reagents were fluoride–extracted.  Results:  The MES data were analyzed using models (i.e., surface complex templates) based on the stoichiometries of hydroxyapatite, calcium-deficient (defective) hydroxyapatite, whitlockite, and octacalcium phosphate.  Conclusions: It can be concluded from the MES data analysis that when 40% or less of the solution calcium was substituted by strontium, the surface complex can best be described by the calcium-deficient hydroxyapatite stoichiometry (Ca₉(HPO₄)(PO₄)₅OH).  When the substitution of solution calcium by strontium was 60% or more, a stoichiometry of Ca₇Sr₂(HPO₄)(PO₄)₅OH best described the experimental data.  Supported by NIDCR Grant DE 06569.