Amphiphilic block copolymers consisting of poly(butadiene) and poly(L-lysine) (PB-P(Lys)) were synthesized and their solution properties studied using dynamic light scattering and transmission electron microscopy. We exploit secondary structure changes that occur within the P(Lys) chain to observe changes in solution morphology as a function of solution conditions. At high pH, the P(Lys) chain assumes either an ƒÑ-helical or a ƒ"-sheet conformation depending on temperature, while at lower pH the side chains become protonated, resulting in an expanded coil configuration. In these studies, three molecular weights and compositions of PB-P(Lys) were studied. For short P(Lys) blocks the block polymers assemble into vesicles that swell with decreasing pH. For longer P(Lys) blocks, cylindrical micelles are formed that undergo a morphological shift to spherical micelles with decreasing pH, or spherical micelles that swell with decreasing pH.