Publisher Summary The chapter describes general methods for rapidly blocking thiol-disulfide exchange and for analyzing cell extracts in ways allowing for the distinction among the oxidized and reduced forms of proteins. A sample labeling protocol specifically designed to detect defects in disulfide bond formation in Escherichia coli ( E. coli) is detailed as one specific example; however, modifications of this protocol should work with many species. In addition, the trapping and separation techniques described have many applications for the study of disulfide catalysis using purified proteins in vitro , and for the study of the folding process itself, in the absence of catalysts. These applications, additional methods, and the theory used to study disulfide exchange have been described in detail. The chapter also describes a generally applicable method to determine the equilibrium redox properties of disulfide catalysts. E. coli mutants that are deficient in thioredoxin reductase and in the disulfide bond catalysts DsbA, B, C, and D show altered abilities in disulfide bond formation in vivo , demonstrating the important role of these folding catalysts in the cell. One advantage of studying these catalysts is that disulfide exchange, both in vitro and in the cell, can be frozen at any instant in time using thiol-trapping agents and the reaction intermediates present within the substrate proteins and catalysts separated and quantified. This provides a unique opportunity to study the kinetics of folding catalysis in vitro and in vivo .