Adsorption of polyethylenimine (PEI) of different sizes on swollen delignified pulp fibers indicates that for PEI molecules of diameter smaller than 25 nm, the accessible internal surface area within the pores in the cell wall is independent of the size of the PEI molecule. This suggests that a minimum pore radius R. exists in the fiber wall (with the possible exception of very small pores of about 1 nm) through which all PEI molecules in the range 2-25 nn can pass freely. Since the molecules must be able to pass through pores with walls fully coated by PEI and since the thickness of an adsorbed PEI layer is comparable to the size of PEI in solution,the pore size must be at least 3 times the size of PEI, implying that R40 mn. A value of the pore radius in the range 45-50 nm is found from estimates of the area of pores accessible to PEI and the corresponding pore volume. No pores are found in the range 3-40 nm. These findings differ from the pore radii obtained by the solute exclusion technique which usually are around 10 nm. The difference might be due to the ease with which the pores contract and expand under different conditions. Non-adsorbing molecules could cause the pores to contract due to depletion effects, while adsorbing molecules might cause pores to expand.