The complex [Re(4,4'-(CO2H)(2)-bpy)(CO)(3)(4,4'-bpy)](CF3SO3), 1 (4,4'-(CO2H)(2)-bpy = 4,4'-dicarboxyl-2,2'-bipyridine, 4,4'-bpy = 4,4'-bipyridine), synthesized and characterized by spectroscopic techniques, displays a strong dependence of its photophysical properties on pH. From both emission intensity and lifetime measurements at different pH values, three values for the protonation constants of the excited states have been determined (pK(a1)* = 1.8 +/-0.1, pK(a2)* = 3.9 +/- 0.1, and pK(a3)* = 5.6 +/- 0.1). The unusual bell-shaped variations of these photophysical properties can be accounted for by the changes of energy level orderings induced by each protonation, as confirmed by time-dependent density functional theory (TD-DFT) calculations. Since the solubility, stability, and dynamic range of pH sensing by 1 have been improved with respect to similar tricarbonylpolypyridylrhenium(I) complexes, we conclude that 1 can be used as an efficient molecular switch of the on-off-on type.