The co-aggregation process of a diblock copolymer poly(ethylene oxide)-block-polybutadiene (PEO-b-PB) and a homopolymer poly(acrylic acid) (PAA) in solutions was studied. The number-average molecular weights of both the PEO and PB blocks are 5100 g/mol; the weight-average molecular weight of PAA is similar to 2000 g/mol. The co-aggregation was induced by adding a PB selective solvent (i.e., alkane or cycloalkane) into the THF solution of the two polymers, with the processes characterized by turbidity, H-1 NMR, dynamic light scattering, and microscopy experiments. During the selective solvent titration, the solution underwent a macro-phase separation that was mainly related to PAA, followed by a micro-phase separation that corresponded to the formation of vesicles with the shell of PB block and the core of PAA/PEO complex. The experimental results indicated that the evolution of interpolymer hydrogen bonding complexation between the PAA and PEO blocks determined the co-aggregation process. The loose and soluble interpolymer complex could be formed at rather low selective solvent content (f). The complexation was enhanced with increasing f resulting in "redissolving" the PAA-rich domains in the blend solutions. Afterwards, the more compact PAA/PEO complex chemically linked with a soluble PB block acted as the building blocks to form the vesicles at higher f. (C) 2008 Elsevier Ltd. All rights reserved.