Recent experimental evidence and theoretical predictions indicate that binary blends of relatively monodisperse diblock copolymers remain miscible if the molecular weight disparity of the constituent copolymers is not too great. In this work, we examine the effect of moderate copolymer polydispersity on both the microstructural characteristics and phase behavior of blends prepared from four compositionally symmetric poly( styrene-b-isoprene) (SI) diblock copolymers ranging in polydispersity ((M) over bar(w)/(M) over bar(n)) from 1.02 to 1.30. Blend periodicities, measured by small-angle X-ray scattering, compare favorably with predictions from a strong segregation theory proposed for lamellar diblock copolymer blends composed of monomolecular copolymers. Transmission electron microscopy, employed to ascertain the real-space morphological characteristics of these blends, reveals that a lamellar-->cylindrical transition occurs in macrophase-separated blends.