A nearly symmetric cis-1,4-polyisoprene-1,2-polybutadiene (1,4-PI-1,2-PB) diblock copolymer was anionically polymerized and partially coupled to form a mixture of diblock and 1,4-PI-1,2-PB-1,4-PI triblock copolymers. This mixture was subsequently hydrogenated and fractionated, yielding a homologous pair of poly(ethylenepropylene)-poly(ethylethylene) (PEP-PEE) diblock and PEP-PEE-PEP triblock copolymers, along with three intermediate mixtures containing both molecular architectures. The orderdisorder transition temperature, TODT, which was determined for each mixture by dynamic mechanical spectroscopy, was found to vary significantly with molecular architecture. For the triblock material TODT was 72 °C higher than that for the homologous diblock specimen, which compares favorably with the 61 °C difference anticipated by mean-field theory. Above TODT the dynamic mechanical properties are insensitive to variations in molecular architecture, while below the order-disorder transition subtle variations in the low-frequency viscoelastic response are apparent.