The competition between proton and hydrogen atom transfer in ammonia clusters is studied in the excited C̃′ state using femtosecond pump-probe spectroscopy. Little effect of solvation is seen for the case of proton transfer, while the hydrogen transfer processes display significant dependence on the degree of clustering. The former have lifetimes from 85 to 135 fs for (NH3)3–40 while the latter display values ranging from 300 to 1500 fs. Similar effects are found for deuterated systems, with a relatively large isotope effect for the atom transfer process. The results are consistent with the formation of ion pairs in the excited state and published findings for related phenomena in isolated ion-molecule reactions [Conaway et al., J. Chem. Phys. 87, 3453 (1987)].