Chitosan (deacetylated chitin), which is a naturally occurring polysaccharide having primary amino groups, is known to be insoluble in water because of its strong intermolecular hydrogen bonds. We prepared water-soluble poly(ethylene glycol) grafted chitosan (PEG-g-chitosan) through the chemical modification of chitosan and investigated its aggregation phenomenon in aqueous solutions. The solution properties of PEG-g-chitosans differ depending on the degree of introduction of PEG in aqueous solution, and were studied by measuring transmittance and light scattering. The PEG-g-chitosans could form aggregates spontaneously by means of their intermolecular hydrogen bonds in the aqueous solution. The PEG-g-chitosan aggregates could also uptake N-phenyl-1-naphthylamine (PNA) as a hydrophobic substance in neutral conditions and this PNA could be released from the aggregates in acidic conditions.