Secondary hydroxylamines, (RCH2)(2)NOH and (R2CH)(2)NOH, were converted to nitrones, RCH2N(O)=CHR and R2CHN(O)=CR2, in >94% yield with hydrogen peroxide as an oxygen donor and methylrhenium trioxide (MTO) as a catalyst. High concentrations of hydrogen peroxide were used so that the methylrhenium diperoxide, CH3Re(O)(eta(2)-O-2)(2)(H2O), was the dominant and reactive form of the catalyst. Representative rate constants are as follows: k/L mol(-1) s(-1) = 150 (R = Me), 52 (Et), 13.8 (Pr-i), and 3.33 (PhCH2) in methanol at 25.0 degrees C. There is no H/D kinetic isotope effect on the rate constant for this step. The data are interpreted to infer the intervention of an oxygenated intermediate, (RCH2)(2)N(O)OH, which then rapidly dehydrates to yield the nitrone. Two products are formed from unsymmetrical hydroxylamines, the ratio of which establishes the reactivities of the intermediate toward the competing elimination reactions: (RCH2)(R'CH2)NOH-->((RCH2)(R'CH2)N(O)OH)-->chi RCH2N(O)=CHR'+(1-chi)R'CH2N(O)=CHR.