The solution properties and the micellization behavior of double hydrophilic diblock copolymers depend on the degree of ionization of the pH-tunable block. This is investigated by dynamic light scattering and H-1 NMR spectroscopy in aqueous solutions of diblock copolymers comprising a neutral hydrophilic poly( hexa(ethylene glycol) methacrylate), PHEGMA, block and an ionizable poly(2-(diethylamino) ethyl methacrylate), PDEAEMA, block. At low pH the copolymer is in its unimer state due to the hydrophilicity of the protonated tertiary amine units, while an increase of the solution pH results in the deprotonation of the amine residues, which become hydrophobic, and leads to the formation of micelles consisting of a PDEAEMA core and a PHEGMA corona. The critical degree of ionization for chain aggregation and micelle formation is determined. It is found that the polymer exists as unimers in the aqueous solution for as low as 30% protonated amine groups, whereas for 20% degree of ionization a slight increase in polymer aggregation is observed. When the fraction of ionized amines decreases further to 10%, hydrated micelles are initially formed, followed by the formation of equilibrium micellar structures only upon the complete deprotonation of the PDEAEMA block which thus becomes fully hydrophobic. This unimer to aggregate to micelle transition is observed in the quantitative analysis of the NMR data at similar degrees of ionization to those obtained by DLS, signifying that NMR spectroscopy can be used to follow the micellization process in block copolymer systems.