We have carried out time-resolved fluorescence quenching experiments, using pyrene derivatives as probes, on aqueous solutions of a hydrophobically modified alkali-soluble emulsion (HASE) polymer 1. This polymer is st 1:1 copolymer of ethyl acrylate and methacrylic acid, containing 1 mol % of a macromonomer with a C20H41 group at the end of an oligo(ethylene oxide) (EO32) spacer. The experiments we describe establish that the polymer 1 at full neutralization in water forms two different types of hydrophobic domains. When various pyrene derivatives are employed as probes, they partition between the two environments and exhibit different fluorescence decay times in the different types of domains. For experiments carried out at elevated probe concentration where excimer formation is important, proper analysis of the data is possible only if the solutions are deaerated. The data are fitted to a series of models invoking the presence of a single hydrophobic domain and two different hydrophobic domains. Only the latter model provides self-consistent results. From this model, we infer that the C20H41 groups form micelles with a mean aggregation number of 55 hydrophobes per micelle.