We investigated the early stages of electroless copper (Cu) deposition onto ruthenium (Ru) using a novel ex situ electrochemical technique involving underpotentially deposited lead (Pb-upd). The fractional surface coverage of Cu during electroless deposition on Ru was measured via the Pb-upd deposition charge. Recognizing that the onset potential for Pb-upd is surface-sensitive, the fraction of the exposed Ru substrate and that of the electroless Cu covered substrate could be quantitatively measured. Our investigation, while providing a convenient and easy-to-use surface analytical method, supports the two-dimensional electrochemical growth mode of Cu during early stages of electroless Cu nucleation on Ru. (C) The Author(s) 2016. Published by ECS. All rights reserved.