In-situ X-ray photoelectron spectroscopy (XPS) studies reveal that plasma cleaning of air-exposed Co or Cu transition metal surfaces results in the formation of a remnant C film 1-3 monolayers thick, which is not reduced upon extensive further plasma exposure. This effect is observed for H-2 or NH3 plasma cleaning of Co, and He or NH3 plasma cleaning of Cu, and is observed with both inductively coupled (ICP) and capacitively-coupled plasma (CCP). Changes in C 1 s XPS spectra indicate that this remnant film formation is accompanied by the formation of carbidic C on Co and of graphitic C on Cu. This is in contrast to published work showing no such remnant carbidic/carbon layer after similar treatments of Si oxynitride surfaces. The observation of the remnant carbidic C film on Co and graphitic film on Cu, but not on silicon oxynitride (SiOxNy), regardless of plasma chemistry or type, indicates that this effect is due to plasma induced secondary electron emission from the metal surface, resulting in transformation of spa adventitious C to either a metal carbide or graphite. These results suggest fundamental limitations to plasma-based surface cleaning procedures on metal surfaces. (C) 2016 Elsevier B.V. All rights reserved.