Two classes of [2Fe-2S] proteins have been defined according to the mean value gav of their g tensor components (Bertrand, P., Guigliarelli, B., Gayda, J.P., Beardwood, P. and Gibson, J.F. (1985) Biochim. Biophys. Acta 831, 261-266). To characterize their magnetic properties better, we have compared the spin-lattice relaxation behavior of typical proteins which belong to these two classes, namely Spirulina maxima and adrenal ferredoxin for the gav approximately 1.96 class, Thermus thermophilus Rieske protein and Pseudomonas putida benzene dioxygenase for the gav approximately 1.91 class. For all these proteins, the data support the existence of an efficient Orbach process in the highest temperature range, which allows the determination of the exchange coupling parameter, J. From the comparison of the J values obtained in each class, it is concluded that the structural factors which determine the value of the g tensor and the strength of the antiferromagnetic exchange interactions are different.