A discussion is given of the problems associated with the measurement, by the beam-foil technique, of the mean lives of excited states of ions. The case in which the transition of interest is isolated using a plane-grating monochromator is considered in detail and the optimum mode of use determined. Results are presented which show that the plural scattering of the ions in the exciter-foil sets a limit to the wavelength resolving-power attainable.