Proton transfer in protonated Nafion fuel cell membranes is studied using several pyrene derivative photoacids. Proton transfer in the center of the Nafion nanoscopic water channels is probed with the highly charged photoacid 8-hydroxypyrene-1,3,6-trisulfonate (HPTS). At high hydration levels, both the time-integrated fluorescence spectrum and the fluorescence kinetics of HPTS permit the determination of hydronium concentration of the interior of the water pools in Nafion. The proton transfer kinetics of HPTS in protonated Nation at maximum hydration are identical to the kinetics displayed by HPTS in a 0.5 M HCl solution. The hydronium concentration near the water interface in Nafion is estimated with rhodamine-6G to be 1.4 M. Excited state proton transfer (ESPT) is followed in the nonpolar side chain regions of Nafion with the photoacid 8-hydroxy-N,N,N',N',N '',N ''-hexamethylpyrene-1,3,6-trisulfonamide (HPTA). Excited state proton transfer of HPTA is possible in protonated Nafion only at the highest hydration level due to a relatively high local pH.