Cyclic voltammetric measurements at different temperatures, concentrations and scan rates give evidence for the reversible dimerization of 3,3 ,5,5 ,tetramethyl-2,2 -bithiophene radical cations. The analysis of the experimental data confirm the mechanistic concept of a RR dimerization. The rate constants and equilibrium constants were determined from working curves and digital simulations. The reaction enthalpy is -60 kJ mol - 1 . The weak temperature dependence of the rate constants for the dimerization results in values of the activation enthalpy that is close to zero or even negative.