The triplet excited state of acridine orange ((3)*AO) undergoes a proton-coupled electron transfer (PCET) reaction with tri-tert-butylphenol ((PhOH)-Ph-ttb) in acetonitrile. Each of the reaction components possesses a spectroscopic signature, providing a rare opportunity to monitor the individual proton transfer, electron transfer, and H-center dot-transfer components in parallel via transient absorption spectroscopy. This enhanced optical tracking, along with excited-state thermochemical analysis, facilitates assignment of the mechanism of excited-state PCET reactivity. (3*)AO is quenched via concerted proton-electron transfer (CPET) from (PhOH)-Ph-ttb to form acridine radical (AOH(center dot)) and (PhO center dot)-Ph-ttb (k(CPET) = 3.7 x 10(8) M-1 s(-1), KIE = 1.3). Subsequently, AOH(center dot) reduces the phenoxyl radical (k(ET) = 5.5 x 10(9) M-1 s(-1)), forming AOH(+) and (PhO-)-Ph-ttb followed by proton transfer (k(PT) = 1.0 x 10(9) M-1 s(-1)) to regenerate the starting reactants.