The present work demonstrates that the use of a nonionic surfactant in the mobile phase together with light scattering coupled to size exclusion chromatography (SEC) provides an accurate determination of macromolecular dimensions of hydrophobically modified water-soluble polymer and polyelectrolyte, i.e., weight-average molar mass M(w) and polydispersity I(p). This method, called micellar SEC, is based on the dissociation of the aggregates in aqueous solution and the formation of mixed micelles between the surfactant and the polymer hydrophobic groups. The methodology and its application are presented for synthetic sulfonated polyacrylamides (5 and 20 mol %) modified with three hydrophobic alkyl side groups (C8, C12, and C18) and with Triton X-100 as a nonionic surfactant and are discussed according to the associativity of polymers. The results are compared to those obtained by classical SEC in 0.1 M NaNO(3) and by static light scattering in formamide solution.