Radiosterilization induces radicals, and it is very important to describe radical mechanisms before the possible use of cephalosporins gamma sterilization. Moreover, physiological or radiotherapeutically induced free radicals also initiate radical mechanisms. For this study, pulse radiolysis was used. This method permits to avoid in vivo direct study difficulties of bioradical processes and gives quantitative data. Reactions of solvated electron (eaq-), hydroxyl radical (.OH), azide radical (N3.), dibromine radical anions (Br2.-), oxygen, and superoxide radical (O2.-) with three cephalosporins have been studied. Absorption spectra and rate constants have been determined. It has been found that both eaq- and .OH quickly react (k congruent to 10(10) mol-1 L s-1) with the molecules to give radicals with similar absorption spectra. N3. gives an absorption spectra that has been attributed to an electron transfer, whereas a part of .OH and Br2.- could add themselves to an unsaturated bond.