Arsenic concentrations are commonly high in thermal fluids associated with active volcanism and shallow magmatism. Our current studies focus on the distribution of As in thermal features of Yellowstone National Park and the processes that affect the redox speciation. Combining historical water analyses (320) with results from the present investigation (70), As concentrations typically range from 0.1-6 mg/L with a mean (± s.d.) of 1.7 ± 2 mg/L for 390 samples. Dissolved As concentrations vary because of physical processes of (1)boiling that causes increases in concentration and (2)mixing of deep hydrothermal waters with shallow ground water that causes decreases in concentration. These processes are inferred from As vs. Cl concentration plots. The concentrations also vary from chemical processes such as rock type and leachability, oxidation, mineral precipitation, and sorption. Nearly 160 samples have been analyzed for As(III/V) and the results indicate that most, but not all, thermal features are dominated by arsenic (III) at the point of discharge. Octopus Spring is an example of a hot spring that discharges only As(V). After discharge the arsenic redox state either remains reduced if H₂S and/or S₂O₃ are present or, in the absence of reducing agents, it rapidly oxidizes to As(V) by microbial catalysis. Data from Nymph Creek, however, may be an exception because Fe concentrations are 20 times higher than As concentrations and it is known that Fe(III) can rapidly oxidize As(III) at low pH. Nymph Creek has a pH of 2.7 and a temperature range of 35-62°C. Under these conditions the Fe must be microbially oxidized, and then Fe(III) could be oxidizing As(III) because the decrease in reduced Fe and As with distance downstream are identical. We postulate that changes in pH, temperature, or the ratio of oxidized to reduced species concentrations can change the dominant oxidation mechanism between biotic and abiotic. Both biotic and abiotic processes are operative in controlling the redox state of As in thermal features and their overflows.