Ternary block copolymer (BCP)-homopolymer (HP) blends offer a simple method for tuning nanostructure sizes to meet application-specific demands. Comprehensive dissipative particle dynamic (DPD) simulations were performed to study the impact of polymer interactions, molecular weight, and HP volume fraction (phi(HP)) on symmetric ternary blend morphological stability and domain spacing. DPD reproduces key features of the experimental phase diagram, including lamellar domain swelling with increasing phi(HP), the formation of an asymmetric bicontinuous microemulsion at a critical HP concentration phi HP*, and macrophase separation with further HP addition. Simulation results matched experimental values for phi HP* and lamellar swelling as a function of HP to BCP chain length ratio, alpha = N-HP/N-BCP. Structural analysis of blends with fixed phi(HP) but varying alpha confirmed that ternary blends follow the wet/dry brush model of domain swelling with the miscibility of HPs and BCPs depending on alpha. Longer HPs concentrate in the center of domains, boosting their swelling efficiencies compared to shorter chains. These results advance our understanding of BCP-HP blend phase behavior and demonstrate the value of DPD for studying polymeric blends. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 794-803