The rate of stabilization of side chains during protein folding has never been carefully studied. Recent developments in labeling proteins with 19F-labeled amino acids coupled with real-time NMR measurements have allowed such measurements to be made. This paper describes the application of this method to the study of several proteins using 6-19F-tryptophan as the reporting group. It is found that these side chains adopt their final stable state at the last stages of the folding process and that the stabilization of side chains into their final conformation is a highly cooperative process. It is also possible to show the presence of intermediates in which the side chains are not correctly packed. The technique should be applicable to many systems.