The dynamics and self-assembly of polystyrene- (PS-) polyisoprene (PI) miktoarm (mixed-arm) block copolymers in n-hexane, a selective solvent for PI, are investigated. The miktoarms present a branched arrangement in the soluble block in the fashion of PI-(PI)2, where two PI blocks are connected by a common end point to a linear PS-PI diblock. It is found that these copolymers self-assemble into spherical micelles having cores composed of the insoluble PS blocks and coronas of the well-solvated PI-(PI)(2) blocks. Micelles formed from the branched polymer amphiphiles are more compact, having smaller sizes than the micelles formed from a linear PS-PI diblock copolymer of similar molecular weight and composition. As the concentration is decreased, the micelles with large aggregation numbers remain stable, only showing changes in the aggregation number. The hydrodynamic sizes and aggregation numbers determined from the micelles formed from miktoarm copolymers differ from theoretical predictions for spherical micelles made of the equivalent linear amphiphilic diblock copolymers. These differences may arise from the arrangements of the branched blocks inside the micellar corona.