The electrical properties of \ensuremath{\alpha}-quaterthiophene, \ensuremath{\alpha}-hexathiophene, and pentacene single crystals have been studied for low iodine dopant concentrations. The electronic and ionic $({\mathrm{I}}_{3}^{\mathrm{\ensuremath{-}}})$ currents have been separated experimentally, providing quantitative information on the ion mobility. The mobility of the ${\mathrm{I}}_{3}^{\mathrm{\ensuremath{-}}}$ ions in these layered structures is strongly anisotropic. Parallel to the molecular layers the mobility is of order of ${10}^{\ensuremath{-}6}$ to ${10}^{\ensuremath{-}7}{\mathrm{cm}}^{2}/\mathrm{V}\mathrm{}\mathrm{s},$ whereas perpendicular to the layers it is smaller by about 4 orders of magnitude. In addition, the activation energy for diffusion parallel to the layers is also much smaller (0.2--0.3 eV) than for diffusion perpendicular to the layers (0.85 eV).