We have used pyrene fluorescent probe experiments to study aqueous solutions of a poly(ethylene oxide) (PEO) of M = 35000 with C16H33 end groups. The end groups were attached by reaction of the PEO with hexadecyl isocyanate, and the polymer was then purified so that essentially all of the polymer in the sample contained 2.0 end groups/polymer molecule. The hydrophobic end groups of this polymer associate in water to form micelle-like structures which serve as solubilization sites for the pyrene molecules. Fluorescence decay profiles for both pyrene and 1-ethylpyrene were fitted to the Poisson quenching model to obtain the parameters n, the mean number of quenchers per micelle, and k, the pseudo-first-order rate constant for the quenching reaction in the micelle. While the data seem relatively well-behaved, we obtain different; values for the end-group aggregation number (N-R) for the two probes. For ethylpyrene, N-R = 16 and k = 10.4 mu s(-1), while for pyrene, N-R = 21 and k = 8.7 mu s(-1). Plots of n versus [Py] or [EtPy] are linear for various polymer concentrations, but exhibit a nonzero intercept in the limit of low quencher concentration. We examine a number of possible explanations for this behavior.