The transport of an organic cation across a 4–5 nm liposome bilayer is observed in real time using second harmonic generation. It is proposed that an electrostatic barrier between the inside and outside of the liposome develops as the cation crosses the bilayer. This would explain why the SHG signal does not approach zero at long times. To test this mechanism, the antibiotic valinomycin, which can transport alkali ions across a phospholipid bilayer, is introduced into the system. It is found that the transport time is reduced by a factor of three from 90 ± 2 s to 30 ± 1 s with 1.25 × 10⁻⁸ M valinomycin concentration, and a factor of fifteen to 6.2 ± 0.2 s with 1.25 × 10⁻⁸ M valinomycin concentration. In addition, the SHG signal approaches zero, which further supports the presence of an electrostatic barrier that can be eliminated by the alkali ion transporter valinomycin.