Calcium ion-selective electrodes based on the ionophore ETH1001 have been used to obtain free calcium ion levels, and hence by data fitting, metal-ligand stability constants in the presence of phthalate, maleate, malonate, oxalate, diglyconate, and tartrate under the conditions of a background ionic strength of 0.1 mol dm(-3) NaCl and (25 +/- 0.5) degrees C. The dissociation constants for the acids and the stability constants for calcium ion-ligand complexes, for comparison with the calcium ion-selective results, were separately determined with pH electrodes under the same conditions.