The aggregation of bovine serum albumin (BSA) in the solid state and in solution was studied to elucidate the effect of water mobility on the aggregation rate and mechanism. The results suggest that the freeze-dried BSA forms covalently-bonded aggregates via disulphide bonding during storage in the moistened solid state and in solution. The aggregation rate largely depended on the water content. The freeze-dried BSA to which a small amount of water was added (in the moistened state) was found to be more liable to aggregation than that in solution. The aggregation rate in in the moistened solid state exhibited a maximum at a very low level of moisture. In contrast, the aggregation rate in solution increased with increasing ratio of water to protein (with decreasing protein concentration). The results suggest that the increased rate is related to the increase in water mobility, as measured by the spin-lattice relaxation time, T1, of water using 17O NMR, with increasing ratio of water to protein.