We have measured the temperature dependence of reflectivity spectra of a single crystal of (C2H5)(2)(CH3)(2)Sb[Pd(dmit)(2)](2) (dmit = 1.3-dithiol-2-thione-4,5-dithiolate), to elucidate the origin of its phase transition at 70 K. A remarkable splitting has been found for the bonding-antibonding excitation of the dimer, [Pd(dmit)(2)](2), in the low-temperature phase. This manifests that the phase transition is due to the charge separation, 2 dimer(-) -> dimer(0) + dimer(2-). This result, supporting our quantum chemical model for a strongly dimerized two-orbital system, gives a clue to incorporate a new spin-charge-orbital interplay in a molecular assembly. (c) 2005 Elsevier B.V. All rights reserved.