<p>We investigated the phase behavior of diblock copolymer AB/homopolymer C blends in concentrated aqueous solutions using a simulated annealing method. Phase diagrams were constructed as a function of the concentration of all polymers (<i>Φ</i>) and the volume fraction of homopolymer (<i>f</i>_C). Rich phase transition sequences were observed, especially reentrant phase transitions, such as lamellae → inverted cylinders → gyroids → lamellae → disorder, for a given <i>Φ</i> with increasing <i>f</i>_C. By analyzing the variations in the average contact numbers between different components and the effective volume fractions of B-domains, we elucidated the mechanisms of the reentrant phase transitions. We found that the strong attraction between B and C leads to the swelling of B-domains upon addition of homopolymer. Concurrently, the solvent preferentially swells the A-domains over the B+C-domains. The competing swelling effects of the solvent and homopolymer on the A-domains and B-domains, respectively, triggered the reentrant phase behavior in the symmetric AB copolymer system upon addition of homopolymer.</p>