This article describes a cost-effective and sensitive approach for quantifying waterborne arsenic based on gas-phase chemiluminescence. The approach centers on the use of laboratory-built instruments-one for laboratory use and one field-deployable version-that can quantify total arsenic as well as individually measure As(III) and As(V). The regulatory limit for arsenic in drinking water is 10 μg/L. The limits of detection of the gas-phase chemiluminescence instruments are well below 1 μg As/L and the linear range extends to >100 μg As/L. Total arsenic analysis using this approach requires 3 min.