The Pockels effect of polar organic solvents and water within the electric double layer (EDL) on an indium–tin–oxide (ITO) electrode is studied to find that water has the largest Pockels coefficient (230 pm V⁻¹), followed in order by methanol (200 pm V⁻¹), ethanol (84 pm V⁻¹), and dimethyl sulfoxide (DMSO) (20 pm V⁻¹). Electrolyte solutions of water and methanol have nearly the same magnitude of Pockels coefficient, while ethanol and DMSO solutions exhibit two and ten times smaller Pockels coefficients than the methanol solution, respectively. The Pockels coefficient scales well with the hydrogen-bond strength (or average cluster size) divided by the solvent viscosity. This suggests that hydrogen bonding and viscosity play crucial roles in the mechanism of the Pockels effect of these liquids.