The magnetic isotope effects of hydrogen and carbon nuclei were investigated on the intramolecular hydrogen atom transfer (HAT) processes of a series of five different aromatic ketone structures. For this purpose, the reaction outcome (product distribution) of labeled (13C- and deuterium labeling) and unlabeled substrates were compared and the differences between both were evaluated (in solution and confined media) with regard to possibly occurring HFC-induced effects. A clear trend in reactivity was observed, where the use of nuclei with high HFC contributions (e.g. 13C) resulted in a (mass-independent) fractionation of the products, which was generally in favor of the cyclization process. The opposite trend was observed for nuclei with lower HFC contribution (e.g. deuterium, in favor of cleavage products). These differences were even more pronounced when zeolites were used as constrained reaction medium instead of homogenous solutions.