We previously showed that the major histocompatibility complex (MHC) class I chaperone tapasin can be detected as a mixed disulfide with the thiol-oxidoreductase ERp57. Here we show that tapasin is a unique and preferred substrate, a substantial majority of which is disulfide-linked to ERp57 within the cell. Tapasin upregulation by interferon-gamma induces sequestration of the vast majority of ERp57 into the MHC class I peptide-loading complex. The rate of tapasin-ERp57 conjugate formation is unaffected by the absence of beta2-microglubulin (beta2m), and is independent of calnexin or calreticulin interactions with monoglucosylated N-linked glycans. The heterodimer forms spontaneously in vitro upon mixing recombinant ERp57 and tapasin. Noncovalent interactions between the native proteins inhibit the reductase activity of the thioredoxin CXXC motif within the N-terminal a domain of ERp57 to maintain its interaction with tapasin. Disruption of these interactions by denaturation allows reduction to proceed. Thus, tapasin association specifically inhibits the escape pathway required for disulfide-bond isomerization within conventional protein substrates, suggesting a specific structural role for ERp57 within the MHC class I peptide-loading complex.