The aggregation behavior of 3,6-O-carboxymethylated chitin (3,6-O-CM-chitin) in aqueous solutions was investigated by viscometry, gel permeation chromatography (GPC), and GPC combined with laser light scattering (GPC-LLS) techniques. 3,6-O-CM-chitin has a strong tendency to form aggregates in NaCl aqueous solutions with the apparent aggregation number (N-ap) of about 27. There were three kinds of aggregates corresponding to different cohesive energies, the aggregates with low cohesive energy were first dissociated at 60degreesC, the aggregates with middle cohesive energy were then dissociated at 80 to 90degreesC, and the aggregates with high cohesive energy were difficult to be disrupted by heating. Decreasing polysaccharide concentration (c(p)) or increasing NaCl concentration (c(s)) reduced the content of the aggregates. At the critical c(p) of 2.5 X 10(-5) g/mL, the aggregates were dissociated into single chains completely. The change of aggregation and disaggregation of 3,6-O-CM-chitin in water-cadoxen mixtures occurred from 0.1 to 0.4 of v(cad) and were irreversible. Intermolecular hydrogen bonding can be ascribed as main driving force for aggregation.