The radical salt [ET](2)[CuCl4] was obtained by chemical oxidation of bis(ethylenedithio)tetrathiafulvalene (ET) with the tetranuclear copper(II) halide cluster [Cu4OCl10](4-). Although a complex mixture of anions forms in solution during the redox reaction, only this product is obtained as large (>3 mm) single crystals. X-ray diffraction analysis determined that the ET molecules stack in the solid state forming dimerized ID chains along the a axis, interleaved by [CuCl4](2-) anions. The ET dimers show very short S center dot center dot center dot S contacts (<3.41 angstrom). The physical properties are dominated by these intradimer ET interactions. The magnetic behavior shows antiferromagnetic coupling with a singlet triplet gap >620 K (430 cm(-1)). The Cu2+ (S = 1/2) centers are magnetically isolated and yield a narrow EPR line in the X-band at g = 2.01. The ET moieties are EPR silent.