The use of digital recording systems for mass spectrometers is rapidly expanding. The design of such systems is rather critical if subtle sources of error are to be avoided. This paper reports the theoretical considerations on which Weichert et al. (1967) based their design. The Fourier transform of the expected mass spectra is predicted, and a suitable transversal filter is constructed. An inadequate bandwidth will result in measurements which are area-sensitive, leading to errors unless the scan rate is extremely well regulated. This situation can also arise through excessive digital filtering as well as through an inadequate measuring system. It follows that Wiener optimum filters are not usually appropriate. The sampling rate represents a compromise between computational efficiency and noise rejection, since the noise is not normally band limited. The problem of resolving-power multiplication is considered.