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Results of drift mobility measurements by the recent method of the authors4) on three common organic polymers are given and compared. At 80 °C the hole mobility in practically amorphous polystyrene (5 × 10−5cm−1V−1). It is 5 orders of magnitude greater than in 95% crystalline polyethylene. It is suggested that this is not due to the differences in chemical structure, but that the characteristic chain folding in polyethylene produces strong localisation. The temperature dependence of the hole mobility in polystyrene and of the electron mobility in polyethylene terephthylate is shown to correspond neither to a single process with constant activation energy nor to Mott's relation log μ = const (−1)/T12). The tentative conclusions drawn from the temperature dependence in4) are revised, and it is suggested that chain-hopping and trap-hopping must be considered separately, the first contributing a weak and the second a strong dependence on temperature.