A series of nearly symmetric anionically polymerized cis-l,4-polyisoprene-l,2-polybutadiene (1,4PI-1,2PB) diblock copolymers were hydrogenated by using a heterogeneous palladium catalyst, producing a model poly(ethylenepropylene)-poly(ethylethylene) (PEP-PEE) diblock copolymer. Differential scanning calorimetry, small-angle X-ray scattering, and observation of flow behavior have been used to establish the phase state of these monodisperse materials which contain approximately 54% by weight 1,4PI or PEP and range from Mw= 33 000 to 111 000. All 1,4PI-1,2PB samples were found to be homogeneous. For PEP-PEE, an upper microphase separation transition (MST) temperature occurs at 91 ± 10 °C for a Mw= 53 600 sample. Decreasing or increasing molecular weight lowers or raises the MST temperature as predicted by theory. These findings are not accounted for by classical theories which describe the mixing thermodynamics of nonpolar polymers solely in terms of a difference in cohesive energy densities.