Phosphorus is found to be a common impurity in many of the iron tools and weapons produced during the pre-Roman and Roman Iron Ages of Britain (600 BC-300 AD). The effects of this impurity on the properties and performance of antiquarian materials is not well understood, however. The authors present the initial findings of an in-depth study of the distribution, chemistry, and effects of phosphorus in Roman British ironwork. For this purpose, two Roman British iron artifacts from the site of Irchester, Northamptonshire, were examined using powerful techniques for archaeological materials analysis that include electron microprobe, secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM) with energy-dispersive x-ray spectroscopy (EDXS) capabilities, and Auger electron spectroscopy (AES). It was found that phosphorus was indeed present in the artifacts. The phosphorus atoms were predominantly segregated at grain boundaries and thus should have led to a lowering of grain boundary cohesion and a degradation in the performance of the tools.