Miscible blends of polyisoprene (PI) and poly(tert-butylstyrene) (PtBS) have been studied over the entire composition range by means of differential scanning calorimetry (DSC) and dielectric spectroscopy (DS) techniques. Blends with two different molecular weight PtBS, M-n = 1300 and M-n = 2300, and a M-n = 2700 PI have been investigated. The molecular weight of all the polymers were chosen in order to be well bellow the entanglement limit for PI. Calorimetric measurements confirm the presence of two component glass transitions (T-g) as previously reported. The comparison of the component's T-g observed by DSC and the segmental dynamics directly measured for PI by DS, evidence the equivalence between the calorimetric effective T-g and the freezing of the segmental relaxation of PI around 10(2) s observed by DS. Fitting of the experimental T-g values to the Lodge-McLeish model gives self-concentration values that depend on the molecular weight of the PtBS. Finally, blends with the lowest PI content exhibit unusual behavior which is discussed in the framework of nonequilibrium effects.