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To get a reliable estimation of the spatial analytical resolution of electron probe microanalysers with field emission electron sources (FEG-EPMA) it is necessary firstly to know the probe diameter, especially for low acceleration voltages, and secondly to calculate the X-ray emitting volume by Monte Carlo (MC) simulations. In the first step of the investigations presented here the probe diameter was determined by recording SE- and BSE-images of a gold-on-carbon high resolution specimen at various acceleration voltages and probe currents. From the rise of the contrast profiles of the gold insulars edges the probe diameter was derived by using the 1 % – 99 % criterion. The obtained probe diameter ranges from 10 nm at 15 kV/10 pA and 20 nm at 5 kV/10 pA to 65 nm @ 5 kV/10 nA. In the next step line scans measured with the FEG-EPMA are compared to corresponding MC-simulations on especially crafted thin layer specimens. The agreement is not well in all cases but MC simulations are suitable to get an approximation of the expected spatial analytical resolution in dependence of the used acceleration voltage, the probe current and the detected X-ray transition. To get useful numbers it is necessary to distinguish between the quantitative and the qualitative spatial analytical resolution. The former refers to applications requiring the quantification of the elements, i.e., the elemental composition is homogenous within the X-ray emitting volume. Therefore, the quantitative resolution is derived from the edge steepness of the line scans using the 1 % – 99 % criterion. At our specimens resolutions suitable for the quantitative analysis down to about 100 nm were observed. The qualitative spatial analytical resolution is evaluated by means of the 16 % – 84 % criterion and applies to the pure recognition of elemental changes in line scans across precipitates or layers whose sizes might be smaller than the X-ray emitting volume. We obtained qualitative spatial analytical resolutions down to several ten nanometres.