A high-speed photographic investigation established the general nature of the explosive interaction between hot molten metal jets and subcooled (75°F) distilled water. The photographic sequences yielded basic information concerning the relationship between extent of material fragmentation and the intensity of explosive action. Prior theories for the explosive causative mechanism were show to be inadequate to explain the observed explosive action. A thermally controlled initiating mechanism is hypothesized and the experimental evidence supports the hypothesis. Dimensional analysis yields a parameter dominated by the thermal characteristics of the jet system.