Understanding of two-phase heat transfer mechanisms on downward-facing hemispherical vessels is crucial during external reactor vessel cooling (ERVC) under severe accident conditions. Film boiling is the predominant heat transfer regime in the initial stages of quenching under these circumstances. In this work, the process of downward-facing film boiling on the outer surface of a hemispherical vessel is studied using high-speed video. High-speed video is a valuable measurement technique because it does not require any invasive sensors that may alter the natural liquid-vapor interface in film boiling. With highspeed video and a few image processing techniques, accurate measurements of film thickness have been made at four different degrees of subcooling (0, 3, 5, and 10 degrees C) and angular locations (0, 14, 28, and 42) on a hemispherical vessel. With increasing subcooling and decreasing angular location, the vapor film thickness has been found to decrease. Average film thickness at 0 degrees C (respectively, 10 degrees C) subcooling and one second after immersion is found to be approximately 2 mm (respectively, 0.5 mm). High-speed videos taken at 650 frames per second (fps) have shown significant oscillations at the liquid-vapor interface during film boiling. Additionally, oscillations in the film thickness and its wave characteristics have been analyzed at the prescribed angular locations and degrees of subcooling. From the visual data, insights regarding the heat transfer behavior of film boiling are obtained. Additionally, the characteristics of the interfacial oscillations have been related to the heat flux distribution. The mechanism for the interfacial oscillations can be attributed to disturbances in the balance between the wall and interfacial heat fluxes, along with the hydrodynamic instability. (C) 2018 Elsevier Ltd. All rights reserved.