A holder for a quartz resonator with an adjustable air-gap excitation electrode (range: 0-300 mum) is described, able to be used in an EQCM that is provided either with a metallic-film or a glued metallic-foil working electrode. An AT-cut piano-convex quartz crystal of 1.8 MHz and Pt-foil of 3 mum thickness were used. Both the frequency shift and the quality factor of the quartz resonator were recorded during the calibration tests. Calibration data by electro-deposition of copper ( in 0.1 M CUSO4-solution) and oxygen (in both acid and basic solutions) on Pt-foil and Au-film electrodes, and at various air-gap widths, are presented. The frequency shifts for the Cu-calibration fit the Sauerbrey model at any air-gap width of the excitation electrode, while those for O-calibration fit only at relatively large values of the air-gap, > 75 mum. At small air-gap widths ( < 25 mum), the O-calibration data deviate from the Sauerbrey model and yield an enhanced mass-sensitivity, up to 15 times higher. The different effects the air-gap width has on the response of the quartz resonator, as to the respective mass-variation of copper and oxygen layers deposited onto its surface, are ascribed to the difference in their features. Oxygen appears as a weakly bound, mono-atomic layer, while copper forms a rigid, strongly bound bulk layer. (C) 2003 Elsevier Ltd. All rights reserved.