Munoniurn atoms are hydrogen isotopes with positive muons rather than protons, deuterons, or tritons as nuclei. Thus, they have one ninth the mass of 'H, and microsecond lifetimes. By using nuclear physics counting techniques, rnuoniurn can be studied in a wide variety of media and its chemical and physical properties used to appraise hydrogen. Results are described for the interaction of muoniurn with acetone, showing two types of kinetic isotope effects, formation of free radicals, evidence for intermolecular "rnuoniurn bonding", and rnicelle-induced enhancements of reaction rate constants.