High-Vinyl Polybutadiene (HVBD)/cis-Polyisoprene (CPI) blends were characterized by Differential Scanning Calorimetry (DSC) and Positron Annihilation Lifetime Spectroscopy (PALS). A single DSC glass transition temperature T-g is observed, whose composition dependence strongly deviates from additivity, and shows an apparent cusp when the weight fraction of HVBD approximate to 0.75. The free-volume hole size, V-h, and the scaled fractional free volume, h(ps)/C, = I3Vh were determined by PALS from the orthopositronium (o-Ps) intensities, I-3, and lifetimes, tau(3), over a temperature range encompassing T-g and the temperature at which "positronium bubble" formation occurs. In the glass, V-h and h(ps)/C are smaller for CPI than for HVBD, but the thermal expansion coefficient for hole volume, alpha(f), is larger in the melt for CPI than for HVBD; thus, an iso-hole volume temperature occurs in these blends at T-iso approximate to -34 degrees C. Above and below T-iso, V-h and h(ps)/C each show a negative departure from additivity. A quantitative interpretation of the cusp in the composition dependence of T-g can be obtained, via a modified analysis of Kovacs, using free-volume quantities from PALS, with the ratio of scaling constants C-CPI/C-HVBD as an adjustable parameter. At high temperatures, the positron bubble size is smaller in CPI than in HVBD. This agrees with the observation that the thermal expansivity of hole volume, and, hence the internal pressure are larger in the equilibrium melt of CPI. The effect of e(+)-irradiation on the o-Ps intensity was investigated. I-3 decreases more rapidly in the melt as T --> T-g, and then more slowly in the glass, suggesting that the effect is due to trapping of radical or ionic species which inhibit o-Ps formation.