A modular, low cost, mass spectrometric data system based on a hybrid analogue-digital preprocessing unit and the British Standard Interface is described. The basic hardware is physically small, has a wide range of capability, and deals directly with the voltage output from electrically recording spectrometers. Mass measurements from single scans at 1:10 000 resolution at 10 s/mass decade show an r.m.s. error of 9 parts in 106, and at 1: 400 resolution and 0.16 s/decade, an r.m.s. error of 0.06 mass units on all peaks ≥ 1/500 full scale. Measured intensity values are accurate to 5% of calculated for peaks > 1%. System and processing considerations, and practical arrangements are detailed.