Most thermal waters issuing in Lassen Volcanic National Park (LVNP) are acidic (pH =3.5), low-Cl (concentrations =30 mg/L) hot springs which are characteristic of vapor-dominated hydrothermal systems and, as such, are not useful for liquid chemical geothermometry. Thermal waters at Drakesbad and in Little Hot Springs Valley, hot spring localities characterized by neutral pH and low Cl containing water, may have equilibrated in shallow aquifers so that temperatures estimated by both the Na-K-Ca and Na-Li geothermometers approach the measured spring temperatures of 65? to 95?C. Waters rich in chloride (>2000 mg/L), such as those at Growler Hot Spring and Morgan Hot Springs, situated south of LVNP, are the most appropriate springs for liquid chemical geothermometry and indicate subsurface temperatures between 220? and 230?C. The chemical and thermal characteristics of these springs may result either from boiling at depth and subsequent mixing with meteoric water or from conductive cooling during lateral flow. In either case ~220? to 230?C thermal water probably originates inside LVNP and flows south to Morgan Hot Springs.