In vivo or in situ nuclear magnetic resonance (NMR) offers a powerful tool to study the degradation of xenobiotics by microorganisms. Most studies reported are based on the use of heteronuclei, and experiments with xenobiotics have been limited because specifically labeled xenobiotics are not commercially available, with the exception of (19)F and (31)P. (1)H NMR is, thus, of great interest in this area. To avoid problems caused by the presence of water and intrinsic metabolite signals, some studies were performed using a deuterated medium or specific detection of protons linked to the (13)C-(15)N enriched pattern. We report here the application of in situ (1)H NMR, performed directly on culture media, to study the metabolism of heterocyclic compounds. In this review, we show that a common pathway is involved in the biodegradation of morpholine, piperidine, and thiomorpholine by Mycobacterium aurum MO1 and Mycobacterium sp. RP1. In all cases, the first step is the cleavage of the C-N bond, which results in an amino acid. Thiomorpholine is first oxidized to sulfoxide before the opening of the ring. The second step is the deamination of the intermediate amino acid, which leads to the formation of a diacid. We have shown that the cleavage of the C-N bond and the oxidation of thiomorpholine are initiated by reactions involving a cytochrome P450.