Dynamic mechanical measurements were conducted over a range of frequencies in the glass transition zone of an epoxidized (25 mol %) polyisoprene (EPI), polychloroprene, and miscible mixtures thereof. The neat polymers had nearly equal glass transition temperatures. Furthermore, they were found to exhibit very similar segmental dynamics, including their segmental relaxation times, functions, and temperature dependencies. Interestingly, however, blends containing a relatively high concentration of the EPI had shorter relaxation times (by up to an order of magnitude) than the neat components; that is, segmental relaxation speeds up upon mixing. This behavior is interpreted in terms of the decrease in intermolecular cooperativity in the mixtures relative to the neat components. The decrease is associated with a large positive excess volume of mixing measured for the blends and the consequent increased unoccupied volume.