Average statistical segment lengths for a series of miscible protonated polyisoprene/deuterated polybutadiene (HPI/DPB) blends with considerable range in vinyl contents and blend ratios are used to determine average statistical segment lengths of the two polydienes in those blends. These blends exhibit a minimum in the plot of blend average statistical segment length versus blend total vinyl content because in blends the average statistical segment length of DPB decreases as both DPB vinyl content and HPI vinyl content increases and the average statistical segment length of HPI increases as both HPI vinyl content and DPB vinyl content increases. In a blend, both polydienes are more flexible than they are as isolated homopolymers. The increased flexibility of both polydienes when miscibly blended is due to immiscibility on a molecular scale, manifesting itself as an increase of free volume and allowing increased segmental mobility and depressed T-g. There is a matrix effect on the flexibility of each polydiene in a miscible blend. The vinyl content of each polydiene influences the average statistical segment length of the other polydiene. In miscible HPI/DPB blends (as well as polydiene homopolymers), the average statistical segment length of each polydiene chain can be viewed not as an inherent physical property describing the chain's flexibility, rather its flexibility is influenced by the vinyl content of the polydiene chains with which it is entangled. (c) 2008 Wiley Periodicals, Inc.