L-2-Methyl-4-carboxy-thiazoline was isolated by ion-exchange chromatography and RP-HPLC from heated solutions of N-acetyl-L-cysteine. The structure was established by NMR-spectroscopy. The formation of thiazolines during the heating of cysteine-containing peptides (glycyl-glycyl-cysteine, glycyl-cysteinyl-glycine, glutathione) was observed by UV and NMR-spectroscopy and measured by cation-exchange chromatography. The rate of formation was dependent on the pH, the water activity and the temperature. In comparison with untreated samples, a characteristic increase of absorption at 260 nm in heated proteins (beta-lactoglobulin, ovalbumin) was noticed. Using NMR-spectroscopy [two-dimensional: hetero nuclear multiple quantum coherence (HMQC) and total correlated spectroscopy (TOCSY)] the formation of thiazolines within heated proteins (beta-lactoglobulin, egg white albumin and bovine serum albumin) could be demonstrated unequivocally. The signals of the methine-proton of thiazolines as well as the coupling constants of the thiazoline protons are very characteristic and not overlaid by signals of protein-bound amino acids.