DSC has been used for the investigation of the phase behavior of poly(styrene-co-tert-butyl acrylate) and poly(styrene-co-n-butyl methacrylate) heteroarm star copolymers. The theory of de la Cruz and Sanchez for the microphase separation of such polymeric species seems to satisfactorily explain the experimental results. In some cases, the samples exhibit a three phase morphology, i.e., phase A, phase B and a pronounced interface, A+B mixed phase, which may be attributed to the influence of the molecular architecture