The microstructure of a model HASE associative polymer in aqueous salt solution is complex and has not been defined until now. On the basis of the results from static and dynamic light scattering studies, a physical model describing the microstructure of a model HASE polymer in aqueous salt solution is proposed. The model HASE polymer contains a copolymer backbone of equal moles of methacrylic acid (MAA) and ethyl acrylate (EA) with 1 mol % of C16H33 hydrophobic modified macromonomer distributed randomly along the backbone. In very dilute aqueous solutions (0.005-0.1 wt %), two decay modes are observed in the relaxation time distribution function. The fast and the slow modes correspond to the translational diffusion of the unimers and the polymer aggregates (consisting of about five polymer chains), respectively. The polymer aggregate is formed by association of the hydrophobic macromonomer via a closed association mechanism. When the polymer concentration increases, the polymer aggregates grow in size. The fractal dimension, d(f), decreases with increasing polymer concentrations, indicating that the aggregates are becoming less compact. At the same time, when the polymer concentration exceeds 0.1 wt %, the unimers are transformed to oligomers consisting of two or more polymer chains.