A quartz crystal microbalance sensor has been developed for the determination of inorganic arsenic species in water. The gold electrode surface was modified by a self-assembled layer of dithiothreitol, and the frequency change of the modified crystal was proportional to the arsenic concentration from 0 to around 50 mu g L-1, a range which spans the current US EPA maximum contaminent level of 10 mu g L-1 in drinking water. As dithiothreitol is capable of reducing arsenate to arsenite, the sensor detects both species. The method was applied to the determination of arsenic in spiked rain, tap, pond and bottled water; recoveries not significantly different from 100% were obtained for a number of spike additions of less than 10 mu g L-1. Arsenic was only detected in the bottled water sample, at a concentration of 8 mu g L-1. This method is simple, fast, and inexpensive compared with other conventional arsenic detection methods, and has the potential to be used in the field.