The phase behavior of (PS-PIB)(2)-s-PAA miktoarm star terpolymers with varying volume fractions of PAA was investigated directly by transmission electron microscopy, atomic force microscopy, and small-angle X-ray scattering, and indirectly by thermogravimetric analysis and degree of water sorption. The microdomains of (PS-PIB)(2)-s-PAA demonstrate a unique and unexpected progression from highly ordered cylinders, to lower ordered spheres, to gyroid structures with increasing PAA content from 6.6 to 47 wt %. Interestingly, the phase behavior in the miktoarm star polymer system is significantly different from that reported previously for the linear counterpart of similar composition (PAA-PS-PIB-PS-PAA), where a steady progression from cylindrical to lamellar morphology was observed with increasing PAA content. At low PAA concentrations, the morphology is driven primarily by the relative solubility of the components, while at high PAA content the molecular architecture dominates. Thermal annealing demonstrated the thermodynamic stability of the morphologies, indicating the potential for design of novel microstructures for specific applications through precise control of architecture, composition, and interaction parameters of the components. (C) 2016 Wiley Periodicals, Inc.