Oxide layers only a few atomic layers thick are important in electronic devices and heterogeneous catalysis. This has led to the need to understand better the evolution of the oxide stoichiometry with thickness in such situations. This paper shows that a high-stability quartz-crystal microbalance can provide absolute measurements of changes in the stoichiometry of oxides as they evolve one molecular layer at a time. In the case of ultra-thin antimony oxides grown on a gold substrate, the compound SbAuO was observed for Sb deposits of less than 1.6 atomic lavers. This oxide acts as a transition layer between the gold and subsequent oxide layers which are Sb2O3.