To study the effects of co-ions on iodide ion transport through a novel anion exchange paper membrane containing trimethylhydroxypropylammonium groups with approximately 0.30 meq. g -1 dry-membrane ion exchange capacity, transmembrane potential, membrane conductance and ionic fluxes were measured in the concentration-cell system for seven kinds of iodine salts (1:1-electrolytes) at 10 -3 to 10 -1 mol dm -3 . For analysis of ion transport, the electroconductive membrane permeability, representing the solution/membrane ion distribution due to electrostatic effects, and the diffusional membrane permeability, representing the migration speed of ions within the membrane phase, concerning ion pair diffusion, and increasing with co-ion penetration and diffusion, were introduced in terms of linear phenomenological equations. Electroconductive membrane permeabilities of iodide ions were found to increase linearly with increasing hydrated co-ion radii; r 2 = 0.855, while diffusional membrane permeabilities of iodide ions increased linearly with increasing crystal co-ion radii; r 2 = 0.899. These results for a paper membrane differ from those of synthetic amphoteric membrane and nanoporus membranes in which ions of both charges penetrate equally, in which membrane permeabilities decreased with increasing co-ion radii. This suggests that iodide ion transport may be partially controlled by co-ion transport dependent on surface cationic charge density rather than ion radii in the anion exchange paper membrane, as the pores are large enough for ions to freely permeate within membrane matrix.