Reactions of thermally processed vitamins and amino acids, in particular, ascorbic acid, nicotinic acid, L-cysteine and L-methionine were investigated in water at 100 °C. For such analysis, in situ quantitative nuclear magnetic resonance spectroscopy (is q 1H NMR) was used which means that the samples were directly analyzed in water. This method has the advantage to be fast and non-invasive, without any extraction process. Under these experimental conditions, there were 18 % of ascorbic acid and 37 % of cysteine remaining after 96 h of reaction. Nicotinic acid and L-methionine were both found to be stable, even after 96 h of thermal processing.