Recent experimental and theoretical efforts suggest that ordered diblock copolymer blends can be approximated as single-component systems of average composition or molecular weight (or both) if the constituent copolymers are closely matched with respect to composition and molecular weight. In this work, Ne explore the phase behavior and morphological characteristics of five series of poly(styrene-b-isoprene) (SI) diblock copolymer blends in which all of the constituent copolymers are compositionally symmetric (50/50 w/w S/I) so that only copolymer molecular weight disparity is varied. Every blend consists of a copolymer with a number-average molecular weight of 120 000 and a second copolymer with a molecular weight between 12 000 and 68 000. Transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) have been performed to identify the phase(s) present, as well as to quantitate lamellar spacings and phase compositions as functions of blend composition, in each series. Results obtained from both techniques are in reasonable favorable agreement with each other and with predictions from a theoretical formalism proposed for strongly segregated lamellar diblock copolymer blends.