The attachment of energy-dispersive analysers to scanning transmission electron microscopes makes possible accurate high spatial resolution microanalysis. This paper quantifies the X-ray spatial resolution obtainable for good sensitivity microanalysis as a function of microscope lens settings and primary electron beam accelerating voltage in a 200 kV scanning transmission electron microscope. It is shown that at 200 kV a resolution of approximately 500 A is possible with a 1500 A thick nickel-iron-chromium alloy specimen. At 100 kV the resolution drops to nearer 1000 A These figures are supported by Monte Carlo calculations of electron trajectories in thin films. Comparisons of experimentally measured and theoretically predicted X-ray intensities suggest a severe loss of signal intensity within the microscope; this loss becomes greater at higher accelerating voltages. The influence on microanalysis of angle of specimen inclination to beam and to the detector is considered and precautions necessary to limit beam drift during 1000 s analyses are discussed. The application of high sensitivity, high resolution microanalysis methods to a grain boundary segregation effect in an austenitic creep resistant steel is described. It is shown that titanium segregates in a zone 1000-1 500 di thick on either side of the grain boundary to a level of approximately three times the matrix concentration.When considering high energy electron beams (i.e. 100

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