The quenching of emission from the metal to ligand charge-transfer (MLCT) state of the copper(I) complex Cu-(dpp)2+(dpp = 2,9-diphenyl-l, 10-phenanthroline) by the uncharged chromium(III) complexes CrL3(L =acetylacetonate, 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate, l,l,l-trifluoro-2,4-pentanedionate) and several nitrobenzene derivatives has been investigated in dichloromethane, chloroform, and tetrahydrofuran solutions as a function of hydrostatic pressure (0.1−250 MPa). The patterns in all three solvents were similar. For those quenchers having reduction potentials E1/2(Q) < −1.6 V (vs the ferrocenium/ferrocene, Fc+/Fc, couple in dichloromethane), the principal bimolecular deactivation mode is energy transfer. In such cases, there is little effect of pressure on the quenching rate constant kq(Δq= ∼0 cm3mol−1). In contrast, substantially more positive E1/2(Q) values lead to kq's approaching diffusion limits and significantly positive ΔV‡qvalues (up to +8 cm3mol−1), reflecting the pressure-induced increases in solution viscosity. For E1/2(Q) ∼ −1.6 V, the dominant pressure-dependent term is electron transfer within the outer-sphere precursor complex, and the resulting charge creation in the transition state leads to substantially negative ΔV‡qvalues approaching −20 cm3mol−1. © 1993, American Chemical Society. All rights reserved.