Static light scattering measurements have been used to determine the molecular mass (65.3 kDa) and second virial coefficient (3.66x10 - 4 mol mL g - 2 ) for the complex between chicken serum haptocorrin (HC) and aquacobalamin (H 2 OCbl). Dynamic light scattering measurements have also been used to determine the translational diffusion coefficient of the H 2 OCbl-HC complex as a function of concentration. From the diffusion coefficient at infinite dilution (4.71x10 - 7 cm 2 s - 1 ), the hydrodynamic radius (45.5 9) and rotational correlation time (85.4 ns) have been calculated. Using the latter, and measured values of the 3 1 P NMR linewidths of the H 2 OCbl-HC complex at several field strengths, a detailed analysis of the 3 1 P nuclear relaxation is possible. The chemical shift anisotropy term from the transverse relaxation equation is determined to be 95.7 ppm, and the average phosphorus-proton internuclear distance is 2.05 9. For comparison to protein-free H 2 OCbl, the chemical shift anisotropy of the phosphorus atom was studied by solid state NMR spectroscopy and the 3 1 P relaxation by solution T 1 measurements. These studies give values of 110.3 ppm for the chemical shift anisotropy term and 2.45 9 for the average phosphorus-proton internuclear distance. The results are consistent with a significant change in the conformation of the H 2 OCbl phosphodiester upon binding to haptocorrin which could be due to a shortening of the axial Co-N bond.