The self-assembly process in aqueous solutions of the methoxyl-poly(ethylene glycol)-block-poly(2-acrylamido-2-methyl-1-propanesulfonic sodium)-block-poly(N-isopropyl acrylamide) (PNIPAAM) triblock copolymer, synthesized via two different atomic transfer radical polymerization methods, namely "one-pot" (P3-sample) and "two-pot" (P2-sample), was studied by various experimental techniques. The "one-pot" procedure leads to a copolymer (P3) where the PNIPAAM block is contaminated with a minor quantity of 2-acrylamido-2-methyl-1-propane sulfonate (AMPS) residuals and this sample does not form micelles over the considered temperature region, but unimers and temperature-induced aggregates coexist in the presence of a small amount of salt. The P2 polymer forms micelles and intermicellar structures, but the former moieties disappear at high temperatures, whereas the latter species contract with increasing temperature. Small-angle neutron scattering results revealed correlation peaks, both for P3 and P2, and no micelle formation for P3, but a pronounced upturn of the scattered intensity at low wavevector values at elevated temperatures for the P2 copolymer. The findings from this study clearly show that the spurious AMPS residuals have a drastic influence on the self-assembly and micelle formation of the triblock copolymer. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 524-534