The compositional heterogeneity of a concentrated aqueous solution of a commercial hydrophobically associating polysaccharide and its precursor has been investigated, using optical and rheometrical characterization. Optical observations proved that the non-associating precursor solution was rather homogeneous, whereas significant compositional heterogeneity was present in the associating one, containing soft polymeric particles with a size of order 100 mum. Linear and non-linear rheological measurements were carried out on the centrifuged and non-centrifuged associating polymer solution in order to study the rheological influence of these objects and their nature. The results tend to prove that the soft polymeric particles are mainly composed of polymer chains bearing the longest hydrophobic groups.