Unexpectedly, all six investigated substances were found to have a significant effect on the permeability and/or partitioning coefficient of the permeant. Depending on the substance, either enhancement or suppression of permeation was observed (by a factor of up to 6x). The membrane partitioning coefficient was influences by a factor of up to 5x, again both upwards and downwards depending on the co-permeant. There was no simple correlation between the BCS class of the investigated drug and its effect on permeation. Specific molecular interactions with the permeant (CF) and/the membrane lipids were therefore likely the cause of permeation modification in each case.