SecB, a remarkable chaperone involved in protein export, binds diverse ligands rapidly with high affinity and low specificity. Site-directed spin labeling and electron paramagnetic resonance spectroscopy were used to investigate the surface of interaction on the export chaperone SecB. We examined SecB in complex with the unfolded precursor form of outer membrane protein OmpA as well as with a truncated version of OmpA that includes the transmembrane domain and lacks both the signal peptide and the periplasmic domain. In addition, we studied the binding of SecB to the unfolded mature form of galactose-binding protein, a soluble periplasmic protein. We have previously used the same strategy to map the binding surface for the precursor of galactose-binding protein. We show that for all ligands tested the patterns of contact are the same.