C-Nitroso compounds represent a class of chemical species with radical spin trapping capacity. They are widely employed to scavenge nitric oxide (NO) in biological and chemical systems and prospectively could be applied to reduce and control NO formation from important chemical processes. In this study, we examined the physicochemical properties of four ortho-substituted aromatic nitroso compounds, 3,5-dibromo-4-nitrosobenzenesulfonate (DBNBS), 4-nitrosobenzenesulfonate (NBS), 3,5-dimethyl-4-nitrosobenzenesulfonate (DMNBS), and 3,5-dichloro-4-nitrosobenzenesulfonate (DCNBS). These compounds, like most C-nitroso spin traps, exist in a monomer-dimer equilibrium, and only the monomeric form is active as a free radical spin scavenger. The influence of the aromatic ring substituent(s) on the dissociation of the dimer to give the monomer was investigated using UV-vis spectrophotometry. We describe the thermodynamic and kinetic parameters associated with the monomer-dimer equilibria, which allow us to provide a mechanistic understanding of NO trapping by C-nitroso compounds.