The degradation of N-substituted polyethylenimines (PEI) in the presence of benzophenone (BzPh) as photoinitiator has been studied in function of the branching of PEI and of the N-substitution by methoxycarbonylmethyl CH3OCOCH2- and by phenyl groups. The reaction kinetics were followed by viscometry of the polymer solutions in dimethylsulfoxide and expressed in terms of S, the number of main chain scission, and, in the cases of copolymers, in terms of L, number average of linkages. The rate constants of chain scissions of branched PEI are one-third higher than those of linear PEI. Copolymers of EI and N-methoxycarbonylmethylethylenimine prepared by substitution of 1-PEI with methyl chloroacetate have apparent rate constants kapp, which decrease with the degree of substitution x and can be expressed by kapp=(1−x)kd+xka where kd and ka are the rate constants of chain scission and chain addition respectively. In the case of copolymers of N-phenyl EI (M1) and N-methoxy-carbonylmethylpropylenimine (M2), kapp depend on the composition of the copolymers (r1=0.86; r2=0.53) and on the monomer dyad distribution.