The rate constant of decomposition of p-methoxy-p′-nitrobenzoyl peroxide (MN-BPO), k′d, in various monosubstituted benzenes was measured. The value of k′dseemed to increase with the polarity of the solvents. Activation energies and activation entropies of the decomposition were determined. When styrene was present, the rate constant of decomposition of MN-BPO, kd, increased markedly by three to five times, except in the case of cyano-and nitrobenzenes.