Physical and chemical parameters were obtained from the crater lake of Santa Ana during a two-year monitoring program between 2000 and 2002. The lake contains cool (20 °C) acid-sulfate-chloride waters with a pH ∼1, SO₄ = 11,000 mg/kg, Cl = 7000 mg/kg, and total dissolved solids concentration = 23,000 mg/kg. A bathymetric survey revealed a shallow lake with a maximum depth of 27 m and a volume of 0.47 million m³. Chemical data obtained from the lake show that the major cations are derived essentially from the congruent dissolution of the basaltic andesite host rock. Thermodynamic modeling shows that the acid waters last equilibrated with the host andesite at low temperature, ∼100 °C. Stable isotopic data of the lake waters indicate that D/H and ¹⁸O/¹⁶O isotopic ratios reflect the combination of evaporation effects at the lake surface and the contribution of deep magmatic fluids. δ³⁴S_HSO4 = 16.3‰ suggests that the main source of dissolved bisulfate ions is magmatic SO₂. No δ¹⁸O equilibrium is observed between water and bisulfate ion, suggesting slow kinetics of the isotopic exchange at the low-temperature environment of the lake. Gas emissions from the fumarolic field increased in May 2000; lake temperature increased to 30 °C, and dissolved chloride and sulfate increased as well. Following this change in activity, deuterium and oxygen isotopic ratios shifted toward heavier compositions due to enhanced evaporation at the lake surface.