pH can be used to tune the self-assembly of cationic polyelectrolyte dendrimers and oppositely charged dyes and to produce particles with a desired shape and size in aqueous solution. We present fundamental insight into the effect of pH on electrostatic self-assembly of poly(amidoamine) dendrimers of generation 4 and di- and trivalent anionic organic dyes. The solution pH is used as a key to turn on the interaction and to control the association by regulating the macroion charge. Stable and well-defined nanoparticles are formed in solution, being more stable at low pH where the dendrimer protonation is complete. Nanoparticle stability was correlated with ζ-potential measurements. We prove that the assemblies are electrostatically stabilized and elucidate the importance of the surface charge density. pH was also used as a key to nanoparticle dimension and shape. For example, smaller particles form at a lower pH. The nanostructures have been characterized using dynamic light scattering and small-angle neutron scattering. A "phase diagram" has been developed for each dye, showing the assembly size, shape, and instability regions dependent on the pH. Overall, a pH-responsive nanoparticle shape is a key step toward the design of novel smart therapeutic carrier systems.