The application of temperature-programmed desorption (TPD) to the characterization of porous, oxide catalysts is discussed. First, the theory underlying TPD from porous materials is presented briefly and used to demonstrate that arguments are made that TPD of reactive probe molecules, especially when used in vacuum with controlled exposures, provides much more useful information about the catalyst, including site densities of the active phase of supported oxide catalysts and reaction pathways. Several examples from the literature are used in demonstration of this fact, including the determination of titania surface area in silica-supported titania catalysts, the determination of Bronsted-acid site densities in solid acids, and the elucidation of the reaction intermediates the reaction of 2-propenol-1-ol in an H-ZSM-5 zeolite.