ERp27 (Endoplasmic Reticulum Protein 27.7kD) is a homologue of protein disulphide isomerase (PDI) localised to the endoplasmic reticulum. ERp27 is predicted to consist of two thioredoxin-fold domains homologous to the non-catalytic b and b′ domains of PDI. The structure in solution of the N-terminal b-like domain of ERp27 was solved using high-resolution NMR data. The structure confirms that it has the thioredoxin fold and that ERp27 is a member of the PDI family. 15N NMR relaxation data were obtained and ModelFree analysis highlighted limited exchange contributions and slow internal motions, and indicated that the domain has an average order parameter S2 of 0.79. Comparison of the single-domain structure determined here with the equivalent domain within full-length ERp27, determined independently by x-ray diffraction, indicated very close agreement. The domain interface inferred from NMR data in solution was much more extensive than that observed in the x-ray structure, suggesting that the domains flex independently and that crystallization selects one specific inter-domain orientation. This led us to apply a new rapid method to simulate the flexibility of the full-length protein, establishing that the domains show considerable freedom to flex (tilt and twist) about the inter-domain linker, consistent with the NMR data.