A potentiometric and UV spectrophotometric study on Ca 2+ −ampicillin and −amoxicillin systems in NaCl aqueous solution is reported. The potentiometric investigation was carried out at different ionic strengths, I = (0.15 to 1) mol kg −1 , and temperatures, T = (288.15 to 310.15) K. The most reliable speciation models include two species, namely, CaLH and CaL for the Ca 2+ −ampicillin system and CaLH 2 and CaLH for the Ca 2+ − amoxicillin system. The spectrophotometric results obtained for both of the systems are fully consistent with potentiometric ones, confirming the formation as well as the stability of the complex species. The dependence of formation constants of the species on ionic strength over the range I = (0.15 to 1) mol kg −1 in NaCl and on temperature over the range T = (288.15 to 310.15) K is also reported. pL 0.5 empirical parameter (i.e., the concentration of ligand required to bind 50% of the metal cation present in the trace) was employed to evaluate the ability of both of the ligands to sequester Ca 2+ under different conditions of pH, ionic strength, and temperature. For example, pL 0.5 = 1.82 and 2.88, for ampicillin and amoxicillin, respectively, were obtained under physiological conditions.