Aggregation behavior in aqueous solution of a series of poly (ethylene glycol) (PEG)-based macromonomers with methacryloyl group as the only hydrophobic segment has been investigated using surface tension, steady-state and time-resolved fluorescence spectroscopy using pyrene as a probe, and small-angle neutron scattering techniques. The general formula of these macromonomers is CH2=C(CH3)-CO-O-E-m-CH3, where E is the ethylene glycol unit and m = 8 (ME8), 18 (ME18), 49 (ME49), and 120 (ME120). The results indicate that a macromonomer with 8 ethylene glycol units forms as an aggregate above a certain critical concentration, which can be defined as critical aggregation concentration. The observed high value of I-1/I-3 in pyrene emission spectra at the interface of these aggregates and the inability to scatter a neutron beam by these aggregates indicate that the hydrophobic cluster formed by this macromonomer is remarkably solvated. ME18 has a tendency to aggregate but others do not form any hydrophobic cluster. The hemopolymerization behaviors of these macromonomers in an aqueous medium at 70 degreesC are consistent with these possibilities.