N-phenyl-N'-sulfinylhydrazine (PhNHNSO) is known to form hydrogen bonded dimers in the solid. Recent experimental studies have shown that the dimerization of PhNHNSO is very sensitive towards the position of a substituent on the aromatic ring. It was found that upon introduction of a substituent in ortho position, dimerization did not occur. However, substitution in meta and para position did not hinder dimer formation.  This study was carried out to understand why certain PhNHNSOs dimerize, while others do not. The effects of substituent and solvent on dimerization were investigated by means of different computational methods. In general, density-functional theory calculations were combined with topological analyses of the electron density.  The relative energies revealed a preference of the syn configuration over other configurations of the unsubstituted monomer. The introduction of substituents in ortho position led to less stable monomers as well as dimers, whereas meta and para substitution had only a small effect on monomer and dimer stabilities. The geometries, frequencies and electron densities of N-H and S=O bonds are more affected by ortho than by meta and para substitution, and the ortho dimers showed weaker intermolecular interactions because of twisted geometries. The observed intermolecular N-H---O and C-H---O interactions in the dimers are characterized as hydrogen and anti-hydrogen bonds, respectively. Increasing solvent polarity weakens the dimers. The topological properties, structural parameters and energies showed an unfavourable dimerization for ortho substituted PhNHNSO monomers, which is consistent with the experimental results