The association of a monodisperse poly(ethylene oxide) 53 ethylene oxide units long and labeled at one end with pyrene (Py-PEO) was monitored as a function of Py-PEO concentration by steady-state and time-resolved fluorescence, surface tension, static (SLS), and dynamic (DLS) light scattering. The excimer fluorescence decays exhibit no rise time for any polymer concentration investigated in this study (> 0.5 g/L), indicating that pyrene aggregates are present in solution and confirming the existence of Py-PEO micelles. No excimer emission could be detected for Py-PEO concentration smaller than 0.1 g/L. The first critical aggregation concentration (CAC1) for the formation of Py-PEO micelles was estimated to equal 0.003 g/L by SLS. The Py-PEO micelles are surface active above a Py-PEO concentration larger than 0.001 g/L (0.4 mu M) where a clear drop in surface tension is observed at concentrations slightly lower than CAC1. At a Py-PEO concentration of 10 g/L, the surface tension plateaus due to the aggregation of Py-PEO micelles. This corresponds to a second critical aggregation concentration referred to as CAC2. The aggregation was probed by DLS. DLS provided the hydrodynamic diameter of the Py-PEO micelles, which was found to equal 7.2 +/- 3.1 nm. Fluorescence was used to determine the aggregation number of the Py-PEO micelles, which was found to equal 20 +/- 2. The combination of the aggregation number found by fluorescence and the hydrodynamic radius found by DLS led to the conclusion that the Py-PEO micelles are made of a hydrophobic pyrene-rich core surrounded by a corona of collapsed PEO chains.