In this paper, it is shown that tertiary amine methacrylate-based diblock copolymers can form both micelles and reverse micelles in aqueous solution at room temperature. This remarkable behavior is due to subtle variations in the hydrophilic-hydrophobic balance of the two block sequences, which are based on 2-(diethylamino)ethyl methacrylate (DEA) and 2-(N-morpholino)ethyl methacrylate (MEMA). H-1 NMR spectroscopy studies indicate that these DEA-MEMA copolymers dissolve molecularly in water at pH 6-7 at 20 degreesC. DEA-core micelles are formed at higher pH, and MEMA-core micelles are formed at pH 6.5 in the presence of 1.0 M Na2SO4. Both types of micelles were characterized in terms of their intensity-average diameters and mean aggregation numbers using dynamic and static light scattering, respectively. In addition, one of the copolymers (60 mol % MEMA, M-w = 30 000, M-w/M-n = 1.05) was selected for a detailed small-angle neutron scattering (SANS) study. SANS data for the micelles were analyzed in terms of a weakly interacting polydisperse sphere model and were in reasonable agreement with the light scattering data. The DEA-core micelles are larger than the MEMA-core micelles, but both light scattering and SANS data indicated that the DEA-core micelles had a lower mean aggregation number than the MEMA-core micelles. This suggests that the MEMA-core micelles are relatively dense and compact compared to the more loosely packed DEA-core micelles.