ESR spectroscopic investigations on the three-spin system (1) have been carried out. The radical (1) has a quartet ground state and associates to form a diamagnetic dimer. The association constant K 263 = (6.7±0.3) × 10 3 1/mole was determined by susceptibility measurements, using ESR spectroscopy. From the temperature dependence of K, a standard enthalpy of association of Δ H 0 = 0 was obtained. Thus, the dimerisation occurs only as a result of an increase in entropy. The low-temperature ESR spectrum of (1) was interpreted with a quantum-mechanical model. The zero-field parameter D′ = 44.1 G was determined by simulation of the experimental spectrum.