In order to estimate the size of the cavity remaining around the heme of the 3A3-microperoxidase 8 (MP8) hemoabzyme, the formation of 3A3-MP8-Fe(II)-nitrosoalkane complexes upon oxidation of N-monosubstituted hydroxylamines was examined. This constituted a new reaction for hemoabzymes and is the first example of fully characterized Fe(II)-metabolite complexes of antibody-porphyrin. Also, via a comparison of the reactions with N-substituted hydroxylamines of various size and hydrophobicity, antibody 3A3 was confirmed to bring about a partial steric hindrance on the distal face of MP8. Subsequently, the influence of the antibody on the stereoselectivity of the S-oxidation of sulfides was examined. Our results showed that MP8 alone and the antibody-MP8 complex catalyze the oxidation of thioanisole by H(2)O(2) and tert-butyl hydroperoxide, following a peroxidase-like two-step oxygen-transfer mechanism involving a radical-cation intermediate. The best system, associating H(2)O(2) as oxidant and 3A3-MP8 as a catalyst, in the presence of 5% tert-butyl alcohol, led to the stereoselective S-oxidation of thioanisole with a 45% enantiomeric excess in favour of the R isomer. This constitutes the highest enantiomeric excess reported to date for the oxidation of sulfides catalyzed by hemoabzymes.