The chemical reactivity of a series of cephalosporins is examined as a function of the substituents at positions 3 and 7. In most cases, the nature of the C7 side chain has a minor influence on the beta-lactam reactivity. But in the case of amino-containing C7 substituents, when intramolecular nucleophilic attack may occur, the reactivity may be greatly increased. The spectroscopic and structural characteristics of the beta-lactam linkage do not correlate with the chemical reactivity of studied compounds. The hydrolysis rates are linked neither with the IR frequency or 13C NMR chemical shift of the carbonyl beta-lactam nor with the geometry of the beta-lactam ring. However, a relationship is confirmed between the beta-lactam ring opening rate and the polarity of the C3-C4 double bond, reflected in the different 13C NMR chemical shifts of those atoms. The results are an experimental verification of the theoretical calculations of Boyd et al. on cephalosporin model compounds, which foresee that a C3 substituent could favor the opening of the beta-lactam cycle by stabilizing a transition state involved in alkaline hydrolysis.