A cold-cathode duoplasmatron ion source for producing beams of Ar+ or O- has been designed for the Cambridge P7-Concept microprobe secondary-ion mass spectrometer. An experimental investigation of the performance of the ion source as a function of arc current, extraction potential, magnetic field, anode to extraction-electrode spacing and, in the case of O-, intermediate electrode bore diameter and lateral offset, is presented. The conditions giving maximum source brightness have been determined. Brightness measurements give 1700 and 96 A cm-2 sr-1 for Ar+ and O-, respectively. Focused probes 4.2 μm in diameter have been produced carrying current densities of 0.1 and 0.007 A cm-2 for Ar+ and O-, respectively.