An experimental and theoretical study of flow and heat transfer regimes during quenching of a heated vertical channel is presented. The objective of the experimental portion of the research was to obtain quantitative data and observations on the reflooding of an annular channel typical to small research reactors. Data were used to assist the formulation of a theoretical model to predict the rate of precursory cooling during reflooding. Tests were carried out at constant inlet pressure using subcooled water as a working fluid. Measurements included inlet and outlet flow conditions, vapor and liquid temperatures along the test section and the volumetric void fraction as a function of distance from the quench front. Surface heat flux was calculated from the fast temperature measurements along the heated surface. The quench front is shown to lie in the transition boiling region which spreads into the dry and wet segments of the surface.