Herein, we investigate the presence of an isotopic effect in fast Li + ion conductors and assess the corresponding frequency dependence of the activation barrier. By using a combination of nuclear magnetic resonance (NMR) spectroscopy and ab initio molecular dynamics (AIMD) simulations, we characterize the temperature-dependent bulk intrinsic transport of the two superionic conductors Li10SnP2S12 and Li6PS5Cl and experimentally quantify an isotope effect in these Li + ion conductors for the first time. Although previous studies in Li + conductors have indicated that lower vibrational frequencies generally result in lower activation barriers, we find that the lower average vibrational frequency of the lithium ions results in a larger activation barrier. This result can be explained by considering the co-dependence of the activation barrier on phonon frequency and critical phonon occupation. Isotope effects may thus provide unique insights into the phonon perspective of ionic transport in these fast Li + ionic conductors.