The small-angle scattering from star diblock copolymers (SDCs) has been calculated using the incompressible random phase approximation (RPA) and methodologies recently developed. The influence of the interaction parameter and incorporation of homopolymer has been explored theoretically for SDCs of deuteriopolybutadiene and polymethylpentadiene with four arms. For the copolymer where the polybutadiene was the outer block of the arm, the scattering over a temperature range from 298 to 418 K has been explored. Fits to the data have been obtained using the random phase expressions providing values of the interaction parameter and radius of gyration of the inner block. The dimensions of the inner block are unaltered from the unperturbed dimensions of the linear polymer of the same degree of polymerisation. The temperature variation of scattered intensity suggests a spinodal temperature for microphase separation of 196 K. Although the scattering of the mixtures of the SDC with either of the homopolymer exhibited features predicted by the random phase approximation theory, data could only be fitted by using unrealistic values of intermolecular and intramolecular interaction parameters or using radius of gyration values of the homopolymer that indicated phase separation between SDC and homopolymer. SDC data provide support for phase separation being the pertinent explanation for these phenomena.