The association of benzene molecules to form dimers has been studied at temperatures between 15 and 123 K in a CRESU (Cinetique de Reactions en Ecoulement Supersonique Uniforme) apparatus with helium as the buffer gas. Second-order rate coefficients (k(2)) for the formation of dimers have been determined for temperatures between 15 and 88 K. The effective third-order rate coefficients (no study of fall off behavior has been undertaken) obtained by dividing the values of k(2) by [He] show a strong negative temperature dependence. Further evidence for the formation of dimers at critical concentrations of benzene has been obtained from a study of the CH + C6H6 reaction and from LIF spectra. Modelling calculations indicate that any systematic errors in the results arising from the formation of higher oligomers are small. Based on a model in which it is assumed that the benzene molecules in the dimer are free to rotate, third-order rate coefficients have been calculated for dimerization and are in good agreement with the experimental values for T greater than or equal to 30 K, where effects of falloff should not be too great. (C) 2000 American Institute of Physics. [S0021-9606(00)01710-4].