The temperature-isotope dependence of proton-coupled electron transfer (PCET) for a noncovalent molecular dyad is reported. The system consists of an excited-state Zn(II) porphyrin that transfers an electron to a naphthalene diimide acceptor through an amidinium-carboxylate interface. Two different isotope effects are observed for variant temperature regimes. A reverse isotope effect (i.e., k(H)/k(D) < 1) is observed as T approaches 120 K (k(H)/k(D) = 0.9, 120 K), whereas a normal isotope effect (i. e., k(H)/k(D) > 1) is recovered as the temperature is increased (k(H)/k(D) = 1.2, 300 K). The transition between these limits is smooth, with a crossover temperature of T similar to 160 K. These observations are in accordance with charge-transfer dynamics that are susceptible to bath-induced fluctuations in the proton coordinate.