The molecular weight of bovine serum albumin at neutral pH in 0.1 M salt solutions was determined with light scattering measurements. Simultaneously, preferential binding of a salt to the protein was calculated. It was found that the protein aggregates in KI and KSCN, but not in KCl, LiCl, and LiBr solutions. Analysis of the sedimentation behavior has confirmed aggregation as a molecular phenomenon, not an artifact.