Protein disulfide isomerase (PDI) catalyzes protein folding linked to disulfide bond formation in secreted proteins. It consists of four major domains, denoted a, b, b′ and a′. The a and a′ domains each contain an active site motif, -CGHC-, which is directly involved in thiol-disulfide exchange reactions during catalysis. The roles of the b and b′ domains and the functional necessity for the multi-domain structure of PDI are unknown. We now demonstrate that full catalytic activity requires the involvement of multiple PDI domains and that the b′ domain has a particularly important role in catalysis. Reconstruction of the PDI molecule from the isolated a and a′ domains results in a progressive increase in catalytic efficiency as further domains are added. These effects are especially significant in the catalysis of disulfide bond rearrangements in folded substrates, for which all the domains of the protein are required for maximum catalytic efficiency. It is likely that all of the domains of PDI participate in substrate binding interactions and that PDI has evolved its multidomain structure as an adaptation that allows it to catalyze transformations involving difficult conformational changes.