At present, solutions for natural convection problems involving discontinuous changes in wall temperature do not exist. This paper reports results of an extensive experimental investigation of the particular case of a vertical flat plate with a step-change in wall temperature. Temperature profiles and local heat transfer coefficients determined with a Mach-Zehnder Interferometer are presented for a wide range of the pertinent parameters. The flow field associated with this problem was studied by means of a series of Tellurium dye experiments in water. Photographs are presented showing representative flow patterns, and the stability of the flow is considered as a function of the ratio of the excess temperature below a discontinuity to the excess temperature above.