Luminescence spectra of trans-ReO2(tmen)(2)Cl (tmen = tetramethylethylenediamine, (H3C)(2)N(CH2)(2)N(CH3)(2)), show pressure-dependent vibronic structure involving the high-frequency O=Re=O stretching mode in the E-3(g)--> (1)A(1g) transition (D-4h point group). Its frequency and the intensity distribution within the progression are both pressure dependent. The ground-state O=Re=O frequency increases from 868 cm(-1) at ambient pressure to 887 cm(-1) at 51 kbar. The influence of pressure on the shape and relative positions of the ground and emitting state potentials is determined.