The electrochemical deposition of Co, Pt and Pt-Co alloy are studied with the electrochemical quartz crystal microbalance (EQCM) on a gold substrate. Co is deposited from acidic sulphate bath containing boric acid. Different processes are identified in this bath. Electrodeposition of Co on Au substrate is observed at potentials above redox potential, underpotential deposition, most probably due to formation of a Co-Au alloy. At more cathodic potentials, below -0.5 V, metallic Co is formed. The film is completely dissolved at positive potentials during the anodic scan, probably mediated by Co(OH)(2). The electrodeposition of platinum from acidic PtCl62- bath occurs below the thermodynamic potential (0.74 V) with almost 100% efficiency. At potentials negative from 0.0 V the efficiency decreases due to parallel water reduction. The codeposition of Co and Pt is also studied in acidic bath. Here, the decrease of pH due to water reduction on Pt deposits gives rise to precipitation of Co(OH)(2), together with the deposition of metallic Pt and Co. The films contain as major component the Pt3Co alloy. (c) 2007 Elsevier B.V. All rights reserved.