© 2016 American Physical Society.Spectral probes for the CH+, OH+, and SiH+ hydride molecular ions that play key roles in astrophysics and plasma processes are presented. The merged-beam technique at the SOLEIL synchrotron was used to record the photoionization (ion yield) spectra of CH+, OH+, and SiH+ and that of their parent atomic ions, in the K-shell and L-shell regions, respectively. Energies and oscillator strengths for the Kα (CH+ and OH+) and Lα (SiH+) transitions were determined from the spectra. Ab initio calculations interpret the experimental data in terms of contributions from ground and excited valence electronic states.