At pH = 1 and 25 degreesC, the Fenton-like reactions of Fe-aq(2+) with hydroperoxorhodium complexes (LRhOOH2+)-O-III (L = (H2O)(NH3)(4), k = 30 M-1 s(-1), and L = L-2 = (H2O)(meso-Me-6-[14]aneN(4)), k = 31 M-1 s(-1)) generate short-lived, reactive intermediates, believed to be the rhodium(IV) species (LRhO2+)-O-IV. In the rapid follow-up steps, these transients oxidize Fe-aq(2+), and the overall reaction has the standard 2:1 [Fe-aq(2+)]/[LRhOOH2+] stoichiometry. Added substrates, such as alcohols, aldehydes, and (NH3)(4)(H2O)RhH2+, compete with Fe-aq(2+) for (LRhO2+)-O-IV, causing the stoichiometry to change to <2:1. Such competition data were used to determine relative reactivities of (NH3)(4)RhO2+ toward CH3OH (1), CD3OH (0.2), C2H5OH (2.7), 2-C3H7OH (3.4), 2-C3D7OH (1.0), CH2O (12.5), C2H5CHO (45), and (NH3)(4)-RhH (2+) (125). The kinetics and products suggest hydrogen atom abstraction for (NH3)(4)RhO2+/alcohol reactions. A short chain reaction observed with C2H5CHO is consistent with both hydrogen atom and hydride transfer. The rate constant for the reaction between Tl-aq(III) and (LRh2+)-Rh-2 is 2.25 x 10(5) M-1 s(-1).