The reaction of N-substituted phenothiazines with certain acceptors (halogenated solvents CHCl3, CH2Br2, CCl4, o-chloranil, AlCl3, SnCl4, concentrated H2SO4, concentrated HNO3 in HClO4) has been studied by EPR spectroscopy. The hyperfine structure in the EPR spectra of the cation radicals is analyzed. To interpret the EPR spectra obtained in terms of the MNDO-PM3 method we carried out a calculation of the electronic structure of phenothiazine cation radicals containing N-CH3 and N-CH2R substituents. In these radical systems, there are significant steric hindrances to conformational rotation of the CH2 substituent around the N-C bond leading to a conformation with magnetically non-equivalent protons in the methylene group.