Reaction mechanisms in atomic layer deposition (ALD) of ruthenium from bis(cyclopentadienyl)ruthenium (RuCp2) and oxygen were studied in situ with a quadrupole mass spectrometer (QMS) and a quartz crystal microbalance (QCM). In addition, QMS was used to study ALD of platinum from (methylcyclopentadienyl) trimethylplatinum (MeCpPtMe3) and oxygen. The QMS studies showed that the reaction by-products H2O and CO2 were released during both the oxygen and the metal precursor pulses. Adsorbed oxygen layer on the metal surface thus oxidizes part of the ligands during the metal precursor pulse. The remaining ligand species become oxidized and a new layer of adsorbed oxygen forms on the surface during the following oxygen pulse. The QCM analysis of the ruthenium process showed a mass decrease during the RuCp2 pulse and a mass increase during the oxygen pulse, which further supports the proposed mechanism. (C) 2003 The Electrochemical Society.