Mononuclear nonheme Mn(III)-peroxo complexes are important intermediates in biology, and for instance take part in the oxygen activation by Photosystem II. Herein, we present work on two novel isomeric biomimetic side-on Mn(III)-peroxo intermediates with bispidine ligand system and reactivity patterns with aldehydes. The complexes are characterized with UV-Visible and mass spectrometric techniques and reaction rates with cyclohexane carboxaldehyde (CCA) have been measured. The reaction gives an unusual regioselectivity switch from aliphatic to aldehyde hydrogen atom abstraction upon deuteration of the substrate leading to the corresponding carboxylic acid product for the latter, while the former gives a deformylation reaction. Mechanistic details are established from kinetic isotope effect studies alongside density functional theory calculations. Thus, replacement of C–H by C–D raises the hydrogen atom abstraction barriers and enables a regioselectivity switch to a competitive pathway that is slightly higher in energy.