The molar absorbance coefficient of ground-state pyrene aggregates (epsilon(E0)(lambda)) in water generated by a short poly(ethylene oxide) chain labeled at a single end with a 1-pyrenemethoxide unit, namely, the polymeric construct Py-1-PEO(2.5K), was determined by a combination of UV-vis absorption and time-resolved fluorescence experiments. Since direct excitation of ground-state pyrene aggregates results in instantaneous excimer emission, whereas excimer formation by diffusive encounters between an excited- and a ground-state pyrene is delayed, acquisition of a pyrene monomer and excimer fluorescence decays with 1 nm increments of the excitation wavelength can probe the subtle effect that the excitation wavelength has on the extent of instantaneous excimer formation. These differences were taken advantage of to determine the fraction f(agg)(f) (lambda) of the total absorbance of the Py-1-PEO(2.5K) solution that is due to ground-state pyrene aggregates. This was achieved by fitting the pyrene monomer and excimer fluorescence decays globally with the model free analysis. For each wavelength, an f(agg)(f) (lambda) value was obtained which was then related to epsilon(E0)(lambda) and f(agg), the molar fraction of aggregated pyrenes that is wavelength independent. These experiments were conducted for three Py-1-PEO(2.5K) concentrations yielding three similar epsilon(E0)(lambda) spectra as expected since epsilon(E0)(lambda) is an intrinsic property of the pyrene aggregates that should not depend on pyrene concentration. The epsilon(E0)(lambda) spectra of the pyrene aggregates of Py-1-PEO(2.5K) in water were substantially broader and red-shifted compared to that of the pyrene monomer. Its measure provides a simple means to calculate the molar fraction of aggregated pyrene units in a Py-1-PEO (2.5K) aqueous solution from the absorption of the solution. This procedure should become widely applicable to determine f(agg) of water-soluble polymers which have been hydrophobically modified with a pyrene derivative and used as models for associative polymers.