(BEDT-TTF)3(ClO4)2 is a typical member of a large group of isostructural compounds formed by the bis(ethylenedithiolo) tetrathiafulvalene (BEDT-TTF) molecule. This subclass of BEDT-TTF compounds generally displays a metal-insulator phase transition when cooled below room temperature. The origin of these phase transitions is not known in these compounds. We present static magnetic susceptibility and ESR data for single crystals of (BEDT-TTF)3(ClO4)2 which suggests that the phase transition is a mean-field-like Peierls transition. The large size of the spin susceptibility indicates that it is considerably enhanced by Coulomb interactions over the corresponding Pauli susceptibility for noninteracting electrons. We estimate a value for the on-site Coulomb interaction U of about 34 eV, compared to an estimated bandwidth 4t of 0.75 eV. © 1986 The American Physical Society.