The association of diblock copolymers with oppositely charged, water-soluble poly(dimethylamino)ethyl methacrylate (AMA) and sodium polymethacrylate (SPM) blocks was studied in aqueous solution as a function of pH and the segment ratio of the two blocks. The size and the structure of these copolymer aggregates were characterized by dynamic light scattering, electrokinetic, and potentiometric measurements. The association process is mainly electrostatically driven. At pH values around the isoelectric point, unlimited aggregation or phase separation takes place. The position of this region depends on the segment ratio of the two blocks. If the pH is moved away from the isoelectric point, the net charge of the copolymer molecules leads to charged molecular assemblies. At pH values higher than 10 or lower than 4, i.e., one of the blocks is entirely uncharged, copolymer assemblies still exist. The solution behavior of the corresponding oppositely charged homopolyelectrolyte mixtures was also studied. The size of the charged homopolymer complexes is smaller than that for copolymers although the length of the homopolymer molecules is higher.