This work reports an experimental and theoretical evaluation of an isotope effect on the ion transport of Li + ions in fast ionic conductors. By comparing different techniques of nuclear magnetic resonance spectroscopy and ab initio molecular dynamics simulations, a lower activation barrier for ionic motion of Li compared to Li can be found. Two entirely different materials, having different compositions and structure-types, were investigated, yet the same trend in activation barrier can be observed. By computationally assessing the vibrational density of states, a common vibrational origin of this isotope effect is proposed. Here, it is the critical phonon occupation numbers that are related to the observed differences in the activation barriers. Isotope effects thus provide a unique opportunity to investigate the importance of phonon vibrations in fast ionic conductors.