Several fluorescence techniques have been applied to characterize the aggregates of styrene-maleic anhydride copolymers (SMA). The formation of hydrophobic microdomains was confirmed by monitoring the I1/I3 ratios of pyrene dissolved in SMA solutions. Fluorescence quenching experiments indicate that protective quenching is occurring. The excimer-to-monomer ratio of di(1-pyrenylmethyl) ether and stoppedflow fluorescence measurements show that the interior of these SMA aggregates is extremely rigid, much more than that of typical surfactant micelles. Fluorescence anisotropy was applied to determine the hydrodynamic radius of these aggregates which was found to equal 1.7 ( 0.2 nm, in qualitative agreement with earlier measurements by small-angle X-ray scattering and small-angle neutron scattering. SMA aggregates have sizes similar to that of typical surfactant micelles, but their interior is frozen.