Ionization constants of three cephalosporin antibiotics, cefetamet (CEF), cefotaxime (CFX) and ceftriaxone (CFTR) are determined using pH-potentiometric titrations at I=0.1 M (NaCl) and t=25 degrees C. Cefetamet and cefotaxime have three ionization groups: carboxylic, amide and aminothiazole. Besides those three, ceftriaxone possesses an hydroxytriazinone group as new and additional ionization center. In acid medium two overlapping acid-base processes are occurring with acidity constants being: pK1 2.93 (COOH) and pK2 3.07 (aminothiazole) for cefetamet, and pK1 2.21 (COOH) and pK2 3.15 (aminothiazole) for cefotaxime. In the case of ceftriaxone the situation is even more complicated, three overlapping processes coexist with pK1 2.37 (COOH), pK2 3.03 (aminothiazole) and pK3 4.21 (hydroxytriazinone). Protolysis of amide group is happening in the alkaline medium as completely separated process from those in acid medium. The acidity constants which correspond to amide group are pK3 10.65 (CEF), pK3 10.87 (CFX) and pK4 10.74 (CFTR). The influence of the C3 substituent on the dissociation process of the neighboring ionization group, particularly carboxylic group, was considered. The differences in acidity of CEF, CFX and CFTR (pK1: 2.93, 2.21 and 2.37, respectively) are likely to be caused by the stereoelectronic properties of substituents in the beta-position to the carboxylic group due to the combined inductive, hyperconjugative and resonance effects.