Polymer properties can be altered via lithium ion doping, whereby adsorbed Li + binds with H 2 O within the polymer chain. However, direct spectroscopic evidence of the tightness of Li + /H 2 O binding in the solid state is limited, and the impact of Li + on polymer sidechain packing is rarely reported. Here, we investigate a polystyrene/H 2 O/LiCl system using solid-state NMR, from which we determined a dipolar coupling of 11.4 kHz between adsorbed Li + and H 2 O protons. This coupling corroborates a model whereby Li + interacts with the oxygen atom in H 2 O via charge affinity, which we believe is the main driving force of Li + binding. We demonstrated the impact of hydrated Li + on sidechain packing and dynamics in polystyrene using proton-detected solid-state NMR. Experimental data and density functional theory (DFT) simulations revealed that the addition of Li + and the increase in the hydration levels of Li + , coupled with aromatic ring binding, change the energy barrier of sidechain packing and dynamics and, consequently, changes the glass transition temperature of polystyrene.