Hydrothermal alteration associated with volcanism can contribute to major elemental trends and mineralogical transformations in soils. The objective of this research was to determine if hydrothermal processes have played an important role in the formation and current chemical and mineralogical characteristics of soils located on a hydrothermally active area in Lassen Volcanic National Park, California. This area hosts the largest active hydrothermal system in the Cascade Range, the result of shallow subvolcanic magmatic-hydrothermal environments. Hydrothermal features include fumaroles (temperatures ~ 160ēC), mudpots and acid-sulfate hot springs. Parent materials consist mostly of pyroxene andesite flows, silicic flows, and silicic pyroclastics. Five pedons were sampled across a temperature-vegetation gradient with temperatures ranging from 16 to 90 ēC at 50-cm depth. Three vegetative states were present across the gradient, barren land, coniferous forest, and meadow. We hypothesized that the distribution and composition of hydrothermal alteration products would be different along this thermal gradient and their presence would decrease with decreasing soil temperature. Genetic horizons from the five pedons were characterized with standard techniques, and soil mineralogy analyzed using selective chemical extraction, total elemental analysis and x-ray diffraction. Soils had sandy clay loam to clay textures and were strongly acidic pH (3-5) and pH increased with increasing distance from the thermal source. Electrical conductivity decreased with distance from the thermal source. In general, enrichment in mobile elements, such as SO4 and Cl, was associated with hydrothermal fluids and volcanic aerosols. Results showed that soil properties were strongly affected by hydrothermal processes along the thermal gradient.