Polycarbonate/poly( -caprolactone) (PC/PCL) blends are found to be miscible when extruded samples are studied without any further thermal treatment. PCL crystallizes in blends containing 60% or less polycarbonate, a component that remains amorphous for all blend compositions under these conditions. Single, broad calorimetric glass transitions together with distinct component dynamics determined by thermally stimulated depolarization current experiments indicate the miscibility of the blends and the existence of different average local compositions. The Lodge-McLeish model is applied to the compositional variation of the two effective glass transition temperatures. Quantitative agreement is obtained for both components by adjusting the self-concentration values to best fit the experimental points. The relevant length for PCL is very close to its Kuhn length, whereas for PC the best fit leads to a slightly shorter characteristic length. It is shown that upon annealing at sufficiently high-temperature PC undergoes crystallization and thereby induces phase segregation in the otherwise amorphous regions of the blends.