This paper is concerned with the design of A-C diblock copolymer surfactants for stabilizing mixtures of weakly segregated A and B homopolymers. Component A was saturated polybutadiene with 89% 1,2-addition, component B was polyisobutylene, and component C was saturated polybutadiene with 63% 1,2-addition. The C-block exhibits attractive interactions with B and repulsive interactions with A. The effect of the molecular weight and concentration of the A-C block copolymer on the phase behavior of critical A/B mixtures was studied by small-angle neutron scattering. We show that organized microphases are obtained when as little as 1 vol % of the A-C copolymer is added to a 50/50 A/B mixture. In contrast, in the well-studied case of A/B/A-B mixtures, 9 vol % of the A-B diblock copolymer is needed to obtain organized microphases in a 50/50 A/B mixture. Self-consistent-field theory (SCFT) calculations, using independently determined Flory-Huggins interaction parameters and statistical segment lengths, predicted the size of the organized microphases within 10% of the experimental value for most temperatures and concentrations of diblock copolymer. The theoretically predicted phase boundary between organized microphases and macrophase separation was in good agreement with experiments.