Because of the current interest in using in-situ resistance measurements for the rapid investigation of thin film reactions and transformations, it is important to understand the various morphologies that the resistance versus temperature curves may assume. Several typical conditions for both planar reactions (as in silicide formation) and lateral transformations (as in the crystallization of amorphous films) have been explored, The characteristic features of the curves, e.g. inflection points, are analyzed for different conditions such as an increase or decrease in resistance. Emphasis is placed on understanding the appearance, or absence of such features, and on their relation to fundamental reaction characteristics such as heat generation.