AbstractThe values of Δε and σ0 in the expression σ = σ0 exp {−Δε/2kT} for the dark conductivity σ, tend to increase together over the whole range of organic semiconductors. They fall into a broad band of values confined roughly between two lines given by log σ0 = αΔε + β, the compensation rule, with log σ0 varying from 5 ± 4 for Δε equal to 1.0 e.v., to 14 ± 2 for Δε = 4.0 e.v. The parameter σ0 is related to the mobility of charge carriers μ, and the present paper attempts a broad qualitative discussion of the factors determining μ. The following mechanisms are considered to apply for the origin and motion of the charge carriers in order of increasing Δε and log σ0: (a) intrinsic bulk thermal generation, transport by hopping over intermolecular barriers, (b) bulk generation, with tunnelling through intermolecular barriers going over to, (c) narrow‐band theory, (d) electron or hole injection from the electrodes into the conductivity band for high energy gap substances. It is shown that process (b) can only account for a restricted range of log σ0, from 4 to 1.0, except perhaps for proteins where log σ0 = 4, may reflect a high mobility along the C0 … HN network. Electron injection may give rise to log σ0 = 1.6 and it seems possible that very much higher apparent values may arise from a temperature‐variable adsorbed film, on the semiconductor or the metal. Somewhat scanty evidence suggests that while atactic polymers may have low Δε values they may also have low values of log σ0. Going over to isotactic polymers may lower Δε somewhat, and increase σ0 by a factor of 10. It is suggested that to achieve high mobility values, e.g., μ = 104 cm.2 v.−1 sec.−1, it will be necessary to develop intermolecular bridging by metal atoms or hydrogen bonds, to assist charge‐carrier transport through the crystal.