Abstract Oxide films of known mean thickness, in the range 10–1000 Å, have been grown on high purity substrates of aluminium, iron, nickel, niobium and tantalum. These samples have been irradiated at normal incidence with low energy electrons and the characteristic oxygen Kα X-ray emission measured with the aid of a soft X-ray spectrometer. Using 1800 eV electrons, all the standards have been studied and the O Kα signal-to-background ratio has been determined under controlled conditions. Evaluation of the resulting calibration curves provides data from which oxide film thickness from about 10 −9 g/cm 2 (submonolayer) to about 10 −5 g/cm 2 can be determined for these specific metal/metal oxide systems with good accuracy. Samples have also been examined using electrons of energy between 525 and 4500 eV. From these results a more general experimental relationship between the O Kα signal-to-background ratio, film thickness and electron energy has been derived. This more general relationship, derived from five oxide systems having cations ranging from Z = 13 to Z = 75, is considered to be applicable (within specified limits) to any simple oxide-on-metal system in the film thickness range 0–150 Å.