It is demonstrated that the conductivity type of organic semiconductors can be modfied by a mechanism analogous to impurity doping. That is, by suitable choice and control of additives, a given organic host may be rendered eithernorptype. Moreover, it is shown that diffusion doping can be employed to formp‐njunctions in a variety of organic host materials. Two examples are discussed in detail to demonstrate the feasibility of doping. First,p‐type tetrathiotetracene (TTT) is shown to becomentype on addition of orthochloronil (OC). The apparent cause is the formation of TTT/OC charge transfer complex molecules within the bulk of the TTT host. Secondly, the normallyn‐type TTT/OC charge transfer complex is shown to become either increasinglyntype orptype on introduction of metal additives.n‐type conduction is achieved with group I and IV metal additives andp‐type conduction with group II and III metal additives. It is also shown that the additives have a significant effect on the conductivity and the activation energy of conduction. Moreover, the changes in the conductivity of TTT, induced by OC additives, are indicative of a tunneling type conduction mechanism. In contrast, the effects due to metal additives can be described by a simple band model. All experiments were performed with compressed powder pellets and the pellet preparation pressure is shown to have a systematic, residual effect on the material properties. The applicability of the doping technique for the preparation of organicp‐njunction diodes is demonstrated by diffusion doping of TTT with OC, TTT/OC complex with Ag and Mg, Perylene with Br, and tetracyano‐quinedymethanol‐quinolinium (TCNQ/QUIN) complex with Ag. Rectification ratios of up to 200 were recorded using, in all cases, compressed powder diodes.