<p>The static scattering properties and phase stability of diblock copolymer/homopolymer blends are strongly influenced by macromolecular architecture and intermolecular interactions. In the present work, a comparative theoretical study of linear and cyclic A–B diblock copolymer/homopolymer C blends was performed within the framework of the Random Phase Approximation (RPA). Particular attention is devoted to the influence of molecular topology on macrophase and microphase separation transitions. The analysis is based on the partial structure factor <i>S</i>_<i>AA</i>(<i>Q</i>), which provides information on concentration fluctuations and structural organization. The effects of several physicochemical parameters, including the normalized Flory–Huggins interaction parameter <i>χN</i>, the composition <i>f</i>, the total copolymer concentration <i>ϕ</i>, and the degrees of polymerization of both the diblock copolymer and the homopolymer, were systematically investigated. The results revealed correlation peaks associated with microphase separation between blocks A and B, together with enhanced long-wavelength concentration fluctuations characteristic of macrophase separation in the presence of homopolymer C. The phase diagrams demonstrated that cyclic copolymers exhibit enhanced thermodynamic stability and a stronger compatibilizing effect than linear systems. These findings highlighted the crucial role of molecular topology in controlling the structural organization and phase stability of diblock copolymer/homopolymer blends.</p>