Multicomponent systems comprising both semiconducting and insulating constituents promise to broaden the technological potential of organic materials due to the fact that, with such architectures, a matrix of highly desirable characteristics may be realized, which is often difficult to obtain with one single species. Reassuringly, field-effect transistors (FETs) comprising active layers of such insulator/semiconductor blends have been demonstrated to approach the device performance of neat poly(3-hexylthiophene) (P3HT), [1][2][3][4][5] which most often has been attributed to segregation of the semiconducting species to the gate dielectric interface. [2,3] Unexpectedly, we find efficient ambipolar charge transport also in the bulk of crystalline/ crystalline P3HT/high-density polyethylene (HDPE) systems -a strong indication that an interpenetrating charge-transport network is generated not only at the thin-film architecture interfaces. This is evidenced by the fact that in time-of-flight (TOF) photoconductivity measurements on such P3HT/HDPE architectures, we observe high hole and electron bulk mobilities of up to 5 Â 10 À3 cm 2 V À1 s À1 even at higher HDPE content. Clearly, the use of crystalline/crystalline blends and corresponding copolymers is, thus, not only restricted to interface devices, such as field-effet transistors, but can also be employed in structures in which good bulk transport is required, broadening the range of applications such materials systems can be utilized for. This is strengthened by the fact that the environmental stability is significantly enhanced when the insulator HDPE is added to the semiconducting P3HT, despite the lack of a vertical-phase-separation mechanisms, which is known to result in ''self-encapsulation'' benefits. [2,3] In fact, both hole and electron charge-carrier mobilities in P3HT:PE blends of different compositions were not significantly affected when stored in air over a period of four months, in strong contrast to neat P3HT architectures, in which charge transport deteriorated within hours.