This article describes a high-resolution quartz crystal microbalance (QCM) with a reproducibility of 0.2 Hz at 6 MHz, provided the frequency is always measured at the same temperature of the microbalance to within 0.1 degrees C. This enables the microbalance to measure a change in mass equivalent to about 0.1 monolayer of oxygen atoms. Due to its high mass sensitivity, the QCM can be used for absolute measurements of the stoichiometry of ultrathin oxide films. An absolute measure of the stoichiometry of ultrathin metal oxides is difficult to obtain by other means. In this technique, one measures the frequency shift after the deposition of about one monolayer of the metal film onto the gold electrode of the quartz crystal, in ultrahigh vacuum. The deposited film is then oxidized to completion in a low pressure of oxygen gas, and the frequency shift measured again, at the same temperature as before. The ratio of the frequency shifts and the atomic masses gives the oxide stoichiometry. Following the complete oxidation of submonolayer amounts of Sb on the polycrystalline gold electrode of the QCM, the ratio of oxygen to antimony atoms was measured to be unity. However, it is suggested that the stoichiometry of the oxide formed may be SbAuO rather than SbO, since the latter does not satisfy valency considerations. The accuracy of the microbalance measurements has been checked using an inductively coupled-plasma mass spectrometer, which can easily measure,; ex situ, the mass of a monolayer deposit with an accuracy of a few percent.