A modified Donnan model is discussed with consideration of physical interactions in the ion sorption by polyamide as a function of the influence of ions in the system. Lyotropic numbers and the salt effect on water-structure changes are considered to describe their influence on the physical interactions between the sorbate and the sorbent. It is found that some electrolytes, such as sulfates and phosphates, can increase the sorption of some negatively charged ions (e.g., acid dyes) on positively charged polyamide if salt concentration is high (e.g., > 0.5 M), after the initial decrease of ion sorption at a low salt concentration range. This result is difficult to explain by the model that treats all interactions other than ionic interaction as a constant. The modified Donnan model explains both the decrease and increase of ion sorption with increasing salt concentration quantitatively.