It has been difficult to obtain directly residue-specific information on side-chain packing during a fast (ms) protein folding reaction. Such information is necessary to determine the extent to which structural changes in different parts of the protein molecule are coupled together in defining the cooperativity of the overall folding transition. In this study, structural changes occurring during the major fast folding reaction of the small protein barstar, have been characterized at the level of individual residue side-chains. A pulsed cysteine-labeling methodology has been employed in conjunction with mass spectrometry. This provides, with ms temporal resolution, direct information on structure formation at ten different locations in barstar during its folding. Cysteine residues located on the surface of native barstar, at four different positions, remain fully solvent accessible throughout the folding process, indicating the absence of any ephemeral non-native structure in which these four cysteine residues get transiently buried. For buried cysteine residues, the rates of the change in cysteine-thiol accessibility to rapid chemical labeling by the thiol reagent methyl methanethiosulfonate, appear to be dependent upon the location of the cysteine residue in the protein, and are different from the rate measured by the change in tryptophan fluorescence. But the rates vary over only a three-fold range. Nevertheless, a comparison of the kinetics of the change in accessibility of the cysteine 3 thiol, to those of the change in the fluorescence of tryptophan 53, as well as of their denaturant dependences, indicate that the major folding reaction comprises more than one step.