Metastable fractions of singly charged ion beams extracted from different ion sources have been determined by measurement of secondary electron spectra which result from slow ion‐induced potential emission from a well‐defined solid surface. Data are presented for Ar+, Kr+, Xe+beams produced by a NIER‐type electron impact source, a ‘‘COLUTRON’’‐type low‐pressure arc plasma source, and a ‘‘DUOPLASMATRON’’‐type magnetically compressed arc plasma source, respectively. The measured metastable fractions are accurate within 25%, and their dependence on various ion source parameters can be well explained with a simple model of metastable ion production and loss kinetics.