Abstract Experimental isotherms have been determined for a series of aqueous n-alkyl carboxylic acids (formate, acetate and propionate) and a series of aqueous alkyl dicarboxylic acids (succinate and adipate) adsorbing onto hydroxyapatite at a constant pH. The isotherms are reversible as determined by adsorption/desorption studies. The isotherms are non-Langmuirian. The average cross-sectional area of an adsorption site, as determined from the plateau of the formate isotherm, is about 20–25 Å 2/site. The data were simulated by a lattice adsorption model. The variables in the model consisted of a free energy of adsorption term for designated moieties of the adsorbate molecules. The results of the simulations are consistent with the view that the succinate and adipate adsorb as a mixture of various flat and vertically oriented configurations and propionate adsorbs predominantly in vertical configurations.