The speciation and spatial distribution of arsenic on rusted steel surfaces affect both measurement and removal approaches. The chemistry of arsenic residing in the rust of ton containers that held the chemical warfare agents bis(2-chloroethyl)sulfide (sulfur mustard) and 2-chlorovinyldichloroarsine (Lewisite) is of particular interest, because while the agents have been decontaminated, residual arsenic could pose a health or environmental risk. The chemistry and distribution of arsenic in rust samples were probed using imaging secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX). Arsenic in the III and or V oxidation state is homogeneously distributed at the very topmost layer of the rust samples and is intimately associated with iron. Sputter depth profiling followed by SIMS and XPS shows As at a depth of several nanometers, in some cases in a reduced form. The SEM/EDX experiments show that As is present at a depth of several micrometers but is inhomogeneously distributed; most locations contained oxidized As at concentrations of a few percent; however, several locations showed very high concentrations of As in the zerovalent form. These results indicate that the rust material must be removed if the steel containers are to be cleared of arsenic.