A modified technique is presented to record and evaluate electron energy loss spectra (EELS) which gives a lateral resolution for chemical analysis in the nanometre range. To achieve such a resolution, considerations about the chromatic aberration and the behaviour of inelastically scattered electrons in magnetic lenses are necessary. The image-forming system was approximated as consisting of four thin lenses for the computer modelling of ray paths. The results led to experimental conditions for nanometre-scale resolution with EELS. The technique was applied to analyse equidistant features first observed in oxygen-contaminated AlSn sputter layers. Bright field transmission electron microscope images of cross-sections show straight dark lines within individual Al crystals. The width of the lines is roughly 5 nm and their separation varies from 50 to 100 nm. The comparison of spectra taken from the lines and from areas between revealed a marked increase in the tin and the oxygen content along the lines. This observation suggests a film growth in the presence of oxygen characterized by alternating regions of pure aluminium and of tin-oxide-enriched areas respectively. Tin acts as the oxygen-trapping species.