The synthesis, molecular structure as well as in situ spectroelectrochemical and electron spin resonance (ESR) studies of 5,7,8,10,15,17,18,20-octaphenyl-21,23-dithiaporphyrin (S2OPP) are presented. The compound investigated can be reversibly oxidised and reduced to yield the corresponding stable monocation and monoanion species, respectively. The experimental ESR spectrum of S2OPP•+ can be simulated taking into account the splitting constants 2xa N = 2.3 G from two dominating equivalent nitrogen nuclei. The computed spin delocalization at the density functional theory (DFT) for the cation is completely different compared to the spin delocalization of the corresponding monoanion. The largely delocalized unpaired spin density over the heteroatoms of dithiaporphyrin macrocycle was confirmed for the S2OPP•– radical anion. The role of the phenyl substitution in the meso- or β-positions of the parent 21,23-dithiaporphyrin S2P was also investigated theoretically and the results are compared with the normal (N4 core) porphyrin molecule.