The normal component of the DC electric field measured on ISEE-1 usually exhibits an enhancement of a few mV/m over both upstream and downstream values at the Earth's bow shock. Using the measured relative velocity between the shock and the spacecraft (from the ISEE-1/2 time delay in the magnetometer data) we can transform the observed E enhancement to a potential drop (Δϕ). For a subcritical shock the potential drop was very close to the measured change of particle kinetic energy (Δϕ≂280 V on Day 330, 1977), while for a supercritical shock the potential drop was only a fraction of the measured change of kinetic energy (Δϕ≂140 V on Day 324, 1977). Overshooting of the potential is indicated in the data by a change in sign of the E enhancement.