Sub-Doppler electronic absorption spectra of 12CO+, 13CO+, O2+, CH+ and H2O+ have been obtained by laser/ion beam methods. A fast beam of mass-selected ions is irradiated coaxially with an argon ion laser, and rotational lines in the electronic transition are Doppler-tuned into resonance with the laser by changing the ion velocity. The transitions are detected through predissociation, collisions or ion-molecule reactions, and the linewidths are normally of order 100MHz