In this paper, we used back-foil scanning X-ray microfluorescence (SXRF) and we examined the sensitivity of the technique for the analysis of very thin overlayers, where electron probe X-ray microanalysis (EPMA) reaches its detection limits. The lateral resolution of back-foil SXRF is also calculated for all the systems used. Both experimental results and Monte-Carlo calculations are used in this respect. Back-foil SXRF used in optimized experimental conditions, is found to be more sensitive than EPMA, especially in the case of very thin overlayers. The lateral resolution of back-foil SXRF is of the order of some micrometers. This is much better than the lateral resolution in conventional XRF and of the same order of magnitude as in EPMA.