Data at 298.2 K for aqueous NaCl solutions containing either D-phenylglycin, D-(p-hydroxy)phenylglycine, 6-amino penicillanic acid (6-APA), amoxicillin or ampicillin, are reported. The mean ionic activity coefficients of NaCl were determined from measurements of the responses of a sodium and a chloride ion-selective electrode. The activity coefficients of the precursors or the antibiotics were calculated from the values of the mean ionic activity coefficients using the exact cross differential relation between them. The correlation of solubility data using the activity coefficients measured in this work shows the same puzzling results previously observed in systems containing amino acids.