H-1 NMR self-diffusion and multifield H-2 spin relaxation measurements were applied in a study of the aggregation of a model associative polymer (MAP) in aqueous solution. The polymer is based on a poly(oxyethylene) backbone with ether-linked alkyl chains ends. The aggregation was studied also in presence of sodium dodecyl sulfate (SDS) at a high concentration. Associative polymers self-assemble in aqueous solution into what is generally believed to be micelle-like aggregates with the hydrophobic end groups situated in the micellar core. In binary solutions of polymer and water such polymer aggregates were found to display an extensive polydispersity which increased with polymer concentration, as based on experimental self-diffusion data. Corresponding measurements on mixed solutions of high SDS concentration, on the other hand, indicate that SDS micelles act as seeds for aggregation, promoting a markedly lower polydispersity for these mixed aggregates. H-2 spin relaxation data on SDS molecules in mixed aggregates were found to be consistent with a two-step motional model for chain reorientation. The fast local anisotropic motions of the SDS molecules inside the aggregates were found not to be particularly influenced by the addition of polymer, whereas the slow isotropic motions of the SDS molecules, such as aggregate tumbling and lateral diffusion of SDS over the curved aggregate surface, were considerably retarded. The increased size of the mixed aggregates upon addition of polymer is probably the main reason for this retardation of reorientational dynamics. However, the motion of the polymer in the mixed aggregates and in single-component polymer aggregates could not be described by the same motional model; instead a three-step motional model had to be applied. The additional very slow motion component was interpreted to be a reptational or exchange motion of the polymer inside or between aggregates.