pH-Tunable nanoscale morphology and self-assembly mechanism of a series of oligo(p-phenyleneethynylene) (OPE)-based bolaamphiphiles featuring poly(ethyleneimine) (PEI) side chains of different length and degree of hydrolysis are described. Protonation and deprotonation of the PEI chains by changing the pH alters the hydrophilic/ hydrophobic balance of the systems and, in turn, the strength of intermolecular interactions between the hydrophobic OPE moieties. Low pH values (3) lead to weak interaction between the OPEs and result in spherical nanoparticles, in which aggregation follows an isodesmic mechanism. In contrast, higher pH values (11) induce deprotonation of the polymer chains and lead to a stronger, cooperative aggregation into anisotropic nanostructures. Our results demonstrate that pH-responsive chains can be exploited as a tool to tune self-assembly mechanisms, which opens exciting possibilities to develop new stimuliresponsive materials.