The behaviour of thin polymer films charged by a negative corona discharge in air can be described quantitatively in terms of diffusional migration and trapping of extrinsic charge carriers injected into the material. Up to a field strength of about 105 V cm–1 charge migration is characterized by a field-independent mobility. At higher fields the mobility becomes an approximately linear function of field strength. In the polymers investigated mobilities range from 10–9 to 10–13 cm2 V–1 s–1, corresponding to diffusion coefficients of 10–10 to 10–14 cm2 s–1. Mobilities increase with the solvent content of the polymer and with temperature. The temperature dependence of mobility is adequately described by an Arrhenius equation with activation energies between 10 and 40 kcal mol–1. Charge trapping is indicated by a persistent residual potential which is a function of the injected charge density, the film thickness and the mobility of the charges. The inherent trapping capability of the material is characterized by a trapping frequency. In a few cases, cross sections of the charge traps could be estimated and were of molecular magnitude (5–13 Å).