The dielectric breakdown of pressurized water and salt solutions subjected to high amplitude electric fields of submicrosecond duration has been investigated. Well‐defined pulses (80 kV, 3 ns rise time, 100 ns duration) have been applied to a gap (0.04–0.21 cm), between Rogowski profile electrodes, containing de‐ionized, nondistilled water; de‐ionized, distilled water; sodium chloride solutions (0.001–1.0 M); or magnesium sulfate solutions (0.01–0.1 M). Breakdown in these liquids has been studied at pressures up to 400 atm. Calibrated voltage dividers situated on the source and load sides of the test gap permitted measurement of the interelectrode potential and the current response. From these measurements, the time lag to breakdown, breakdown voltage, power input to the liquid, and temporal characteristics of the breakdown process have been determined. The breakdown time lag increases with increasing pressure and gap width, and decreases with increasing field. Moreover, it is weakly dependent on the conductivity of the liquid. A dynamical model has been developed to explain these results. In this model, field emission currents heat the liquid and create a region with density below the critical density for the formation of electron avalanches. An ionizing wave front subsequently develops and propagates via a sequence of processes occurring in the region ahead of the front; namely, heating by electron injection, lowering of the liquid density, and avalanche growth and retardation. The time for nucleation of the low density region has been experimentally determined and is in good agreement with theory. ©1995 American Institute of Physics.