A bound state of positron (e+) and electron (e-) (positronium Ps) can be considered as a kind of hydrogen-like atom and the simplest free radical, which has a positron instead of a heavy nucleus. Annihilation g-radiation is in fact a label of this short-lived atom. Therefore, positronium chemistry is radiochemistry of the lightest short-lived labeled atom. Application of positronium chemistry to various chemical problems is essentially dependent on our knowledge of analogies and differences in the formation and chemical conversion of Ps, a atomic hydrogen, and other free radicals. In this review, experimental results of the authors and some other publications are briefly analyzed from this point of view.