In order to develop a new chemical mechanical polishing process for ruthenium ( Ru), the present work deals with the effect of a ceric ammonium nitrate ( CAN) additive on the electrochemical behaviour of physical vapour deposited Ru films in a 1 M HNO3 solution employing electrochemical methods and surface analytical techniques. By adding CAN to HNO3 solution, the polarisation curves showed an increase in the corrosion potential and current, suggesting that Ru is anodically polarised by CAN as an oxidising additive. To characterise the influence of CAN, open- circuit potential ( OCP) and potentiostatic anodic current transient curves were examined in CAN-containing HNO3 solution and the resulting surfaces were then characterised by scanning electron microscopy and X- ray photoelectron spectroscopy. It is proposed that Ru is oxidised to heterogeneous Ru2O3 and RuO2 films on the Ru surface in CAN- containing HNO3 solution and galvanic corrosion occurs at grain boundaries, caused by the difference in OCP between the grain interiors and boundaries. The grain boundaries are oxidised to RuO4, a volatile species, resulting in a roughened and porous structure.