Ellipsometry is used to measure the crystallization and melting temperature of a bidisperse blend of a crystalline-amorphous diblock copolymer. Binary blends of sphere-forming poly(butadiene-ethylene oxide) (PB-PEO) of two different molecular weights are prepared. The two PB-PEO diblocks that are used share the same amorphous majority PB block length but different crystalline PEO minority block length. As the concentration of higher molecular weight diblock in the blend is increased, the size of the PEO spherical domains swell, providing access to the full range of domain sizes between the limits of the two neat diblock components. The change in domain size is consistent with a monotonic change in both the crystallization and melting temperatures. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 712-716, 2011