Four amphiphilic block copolymers polyisobutylene-block-poly(methacrylic acid) (IBm-MAA(n); m = 70-134, n = 52-228) were synthesized and transferred into aqueous medium at pH 10-12. Their structure in solution was characterized by fluorescence correlation spectroscopy (FCS), static and dynamic Light scattering (SLS, DLS), analytical ultracentrifuge (AUC), and by transmission electron microscopy (TEM) with freeze-fracturing and staining techniques. DLS data, AUC sedimentation traces, and TEM images indicate at least two different kinds of particles. TEM shows spherical micelles; however, especially for polymers with larger hydrophobic blocks, additional particles are observed. FCS shows extremely low critical micelle concentrations (cmc < 0.3 mg/L). The main part of the particles consists of micelles with diameters from 15 to 50 nm,built by 130-200 block copolymer molecules. Aggregation numbers and diameters are consistent with a model recently proposed by Forster et al. (J. Chem. Phys. 1996, 104, 9956-9970). The packing densities are determined from the hydrodynamic diameters and the aggregation numbers; they vary between 6 and 32%. For large hydrophobic block lengths additional structures are found, in most cases with a narrow size distribution. The origin of these structures is discussed.