Block ionomer complexes (BICs) are prepared from anionic block copolymers and cationic surfactants of different structure or from their mixtures. Drastic changes in the morphology and stability of BIC nanoparticles caused by changes in the composition of the surfactant mixture are demonstrated. Single-tail and double-tail surfactants appear to mix within the BIC, resulting in the formation of rather uniform BIC particles. Morphologies of the particles of these mixed BICs are intermediate between those prepared from pure single- and double-tail surfactants. Particles of BIC prepared from mixtures of single- and triple-tail surfactants are heterogeneous, and FRET experiments indicate that surfactant components in these systems are strongly segregated. The results of this study provide important insights into the formation and structure of the BIC and have implications for various applications of the BIC (e.g., nanomedicine), in which precise control of the shape, size, and other properties is needed.