Observations of impurity behaviour are presented from ICRF heating experiments at 180 MHz performed for a variety of conditions on the Alcator C tokamak, using graphite limiters and stainless steel antenna Faraday shields. Spectroscopic observations revealed significant increases in metal impurity concentrations during the RF pulse, with iron levels increasing by as much as a factor of twelve at the highest RF powers (about 350–400 kW). Analysis of the inferred iron source rates shows an approximately linear dependence on RF power up to 400 kW, with no clear dependence on resonance conditions or bulk plasma parameters. However, a sharp temperature increase in the limiter shadow region was observed during the ICRF pulse, which was well correlated with the iron influx rate. From this and other evidence it is concluded that physical sputtering of the Faraday shield due to an elevated sheath potential is the primary source of metal impurities during ICRF heating on Alcator C. The same process, occurring at the graphite limiter, is believed to be the dominant source of carbon and oxygen. The calculated sputtering yields obtained from an edge erosion code demonstrate the plausibility of this model.