The isotope effect characterized herein is the apparent increase in the activation barrier for the heavier isotope relative to the lighter isotope. That is, the difference Δ𝐸 A = 𝐸 A ( 7 Li)-𝐸 A ( 6 Li) was found to be positive for the investigated fast-conducting sulfide electrolytes (Figure ). This particular isotope effect, Δ𝐸 A , is known to be subtle, and is often neglected in textbook discussions of isotope effects on ionic transport. Yet, we feel that it is especially relevant in contemporary investigations of phonon dynamics in ionic conductors. An additional finding of this study is that the magnitude of Δ𝐸 A appears to increase as the magnitudes of 𝐸 A increase (Figure ). This dependence of Δ𝐸 A on 𝐸 A seems unexpected. Nevertheless, it provides a convenient additional functional dependence with which we can test phonon-based theories of the activation barrier.