We have studied the e.s.r. spectra of methanol solutions of radical cations of the form (1; R = H, Me, Prn, Bun, or PhCH2), over a range of temperature from +40 to –90 °C, and determined the values of the splitting constants. It is found that as the temperature decreases, the concentration of the radical cation decreases, until at –90 °C there is practically no e.s.r. spectrum. This process is reversible. Concentration experiments show that the radical cations are in equilibrium with a dimeric diamagnetic species. The thermodynamic constants for the dimerisation equilibrium were determined, and the results indicate that dimerisation involves desolvation, suggesting that in the dimer the radical cations are arranged in a plane to plane configuration, the two monomer molecules being linked together by a π–π′ bond. For (1; RCH3), i.e. for the radical cation of paraquat, reduction of the temperature is also accompanied by the production of a second paramagnetic species, the structure of which has not yet been determined.