Near-surface, low-temperature, K-metasomatism of rhyolitic to dacitic ash-flow tuffs is a volumetrically and petrologically significant feature of the western United States and may be a primary alteration path in arid lands. Cathodoluminescence (CL) petrography of K-metasomatized ash-flow tuffs quickly yields information about the mineralogical changes and reaction mechanisms that take place during alteration for large sample suites and links these properties to geochemical changes. Tertiary volcanic rocks from Creede (Colorado), Socorro (New Mexico), and the Harcuvar Mountains (Arizona), were altered in different geologic settings, yet have very similar CL textures and chemistry. Original igneous feldspars were replaced by adularia primarily by dissolution and crystallization that occurred preferentially along preexisting fractures, surfaces, compositional zoning features, and cleavages. Fluid flow was primarily along grain boundaries, but was in some cases inhomogeneous at the thin-section scale.