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The metal-to-ligand charge transfer excited states of [Ru(bpy)(3)](2+) (bpy = 2,2′-bipyridine) may be deactivated via energy transfer or electron transfer with ferrocene derivatives in aqueous conditions. Stern–Volmer quenching analysis revealed that the rate constant for [Ru(bpy)(3)](2+) excited-state quenching depends on solution pH when a ferrocenyl-amidinium derivative (Fc–am) containing a proton-responsive functionality tethered to the ferrocene center was present. By contrast, the rate constant with which the [Ru(bpy)(3)](2+) excited state is quenched by an analogous ferrocene derivative (ferrocenyltrimethylammonium, Fc–mam) that lacks a protonic group does not depend on pH. These results show that the presence (or absence) of a readily transferrable proton modulates quenching rate constants in bimolecular events involving [Ru(bpy)(3)](2+) and ferrocene. More surprisingly, transient absorption spectroscopy reveals that the mechanism by which the [Ru(bpy)(3)](2+) excited state is quenched by Fc–am appears to be modulated by solution proton availability, switching from energy transfer at low pH when Fc–am is protonated, to electron transfer at high pH when Fc–am is deprotonated. The mechanistic switching that is observed for this system cannot be aptly explained using a simple driving force dependence argument, suggesting that more subtle factors dictate the pathway by which the [Ru(bpy)(3)](2+) excited state is deactivated by ferrocene in aqueous solutions. [Image: see text]