The segmental motion of each species in polyisoprene/poly(vinylethylene) (PI/PVE) miscible blends is studied at three different compositions using two-dimensional deuteron exchange NMR (2D H-2 NMR). The individual species exhibit widely different mean mobilities and broad mobility distributions near the glass transition of each blend. As the PVE content increases, both the difference in mean mobilities between the two species and the width of the mobility distribution for both components increase. The change in these two types of dynamic heterogeneity with PVE content appears to produce the anomalous broadening of the glass transition. The mean reorientational correlation times of each component can differe by 2 orders of magnitude under identical conditions. This difference can be described in terms of distinct effective glass transition temperatures, T(g)*, for the two species. The separation between the two effective glass transition temperatures increases almost monotonically with PVE content, consistent with the more pronounced thermorheological complexity of blends rich in PVE. The individual T(g)*’s also exhibit a different compositional dependence from that of the calorimetric T(g) of the blend observed by differential scanning calorimetry (DSC). This behavior can give rise to the complex compositional dependence of individual mobilities, apparent when the mobilities are compared at the same T - T(g) with respect to the DSC T(g) of the blend.