Small-scale experiments have been conducted to investigate the triggering mechanism of vapor explosions. In order to attain good repeatability and visibility, a smooth round water droplet was impinged onto a molten alloy surface. This configuration suppresses premixing events prior to triggering. The effect of the water droplet curvature was found to be negligibly small when the droplet diameter is larger than 4.5 mm. Vapor explosion conditions were identical for the molten tin pool depths ranging from 0.5 to 40 mm. The experimental results and the heat conduction analysis suggest that the length scale required for atomizing and fine mixing in the triggering event of the vapor explosion are sufficiently smaller than the molten tin pool depth of 0.5 mm. Six different kinds of materials were used as the pool liquid. The lower limit of the contact temperature in the vapor explosion region closely agrees with the spontaneous nucleation temperature of water. The upper limit of the initial molten alloy temperature decreases when an oxide layer forms on the surface causing an increase of the emissivity of thermal radiation that has a stabilizing effect on the vapor film. When an oxide layer formed on the surface, a water droplet was occasionally entrapped into a molten alloy dome, since the oxide layer prevents the droplet from evaporating coherently. The vapor explosion region obtained for the mirror surface is a conservative estimate, since that for the oxide surface fell into the internal region of mirror surface.