The application of microwave heating technology to chemical processes outside of the food industries has been limited by an incomplete understanding of the temperature distributions formed within a microwave-heated cavity. A fluorescence-based optical method of thermometry is described for the in situ measurement of temperature distributions during microwave heating processes. Highly nonuniform, two-dimensional (2-D) temperature-distribution profiles were measured by resolving spatial variations in the fluorescence response of a temperature sensitive dye, Rhodamine B. The dye was excited by illuminating a slice within the sample cell using a planar expanded laser beam. Significant variations in the heating profiles of water and of salt (NaCl) solutions of different concentrations were observed, consistent with the influence of salt concentration on the dielectric properties of the solutions. The experimentally observed spatial variations in temperature before the onset of convection were in good agreement with predictions using a simple model to describe the electric field distributions in the microwave cavity. (c) 2006 American Institute of Chemical Engineers AIChE J, 52: 2727-2735, 2006.