A major challenge toward realizing high-performance aqueous lithium batteries (ALBs) is the utilization of a metallic lithium anode. However, an ideal solid electrolyte that can protect metallic lithium from reacting with aqueous solutions while still maintaining a high lithium ion conduction is not currently available. One obstacle is the lack of a reliable experimental tool to differentiate the conduction behaviour of H+ and Li+ ions in a solid electrolyte. Here, by correlating neutron and electron spectroscopy, we quantitatively reveal the mobility and lattice occupancy of the two ions individually in protonated cubic Li6.25Al0.25La3Zr2O12 (LLZO). Our results not only highlight LLZO as a potential effective separation layer for ALBs but also present a robust method to quantify the mobility of individual mobile ions in solid-state ion conductors.