The binding behavior of SDS onto several model hydrophobically modified alkali-soluble emulsion (HASE) polymers with different spacer chain length was studied using the isothermal titration calorimetric technique. The titration was performed by injecting 0.1 M SDS into 0.1 wt % HASE polymer solution, which yielded a negative enthalpy and a large positive entropy, thus confirming that the interactions between SDS and HASE polymer is entropy-driven. At low degree of ethoxylation (0-5 mol), HASE polymers with short spacer chains form a type I structure consisting of hydrophobic cross-links and hydrophobe-EA junctions. As the length of the spacer chain increases to 10 mol of ethoxylation, the hydrophobes are more accessible to form interchain junctions, yielding a type II cluster. Associative junctions in type II clusters are more accessible to surfactant than those of type I. With increasing spacer chain length, the critical aggregation concentration (cac) and the Delta G of aggregation decrease, while Delta H remains unchanged. A physical model describing the interaction mechanisms between the polymer and surfactant is proposed.