Volcanic rocks often exhibit internal heterogeneity in radiogenic isotopes. Isotopic disequilibrium between coexisting phenocrysts and isotopic zoning within single crystals has been demonstrated in basalts, andesites, dacites, and rhyolites. High-temperature Snake River–type rhyolites appear to be an exception. Despite the occurrence of Snake River Plain rhyolites in a region of isotopically highly variable crust and mantle, and significant differences from rhyolite unit to rhyolite unit, little to no Sr isotopic zoning is found within their feldspar phenocrysts, and feldspars within a single unit define tightly grouped unimodal populations. High-temperature rhyolitic magmas possess a unique combination of temperature and melt viscosity. Although typically 200 °C hotter than common rhyolites, the temperature effect on viscosity is offset by lower water contents (<3.5 wt%), so melt viscosities are in the same range as common, water-rich, cool rhyolites (105–106 Pa s). However, magmatic temperatures are in the same range as basaltic andesites and andesites; consequently, cation diffusion rates are orders of magnitude greater than common rhyolites. We hypothesize that this combination of characteristics promotes crystal isotopic homogeneity: viscosities are too high to permit crystal transfer between liquids of contrasting 87Sr/86Sr on time scales shorter than those required for diffusive homogenization of Sr within phenocrysts (500–10,000 yr). This is not true for most magma types, in which crystal transfer is rapid (<<100 yr) due to low melt viscosities (basalts and intermediate magmas), or Sr diffusion rates are so slow that equilibration times are longer than the lifetime of the system (e.g., cool, wet rhyolites: 105–106 yr).