In the simulations, we vary the system salt concentration by adding extra monovalent ion pairs to the system and analyze how the microgel swelling changes. In Fig. , we observe microgel swelling relative to the salt-free size versus system salt concentration for the indicated ionization degrees at constant microgel concentration. Notice that the data is vertically shift by 0.1 units for the different ionization degrees to favor a better visualization. For the three cases, we observe a similar decrease of the microgel swelling ratio as salt concentration increases as it is expected from experiments. When comparing against the theoretical models, we first must notice that the theory is formulated within a semi-grand canonical ensemble, with the system open for microions in contact with a microion reservoir of fixed concentration. Due to the Donnan potential, the system salt concentration n sys ≤ n res depending on the different species concentration. Notice that the Donnan potential hinges on microgel swelling. Therefore, the system salt concentration is determined within the theory from the coion concentration at equilibrium swelling, see Fig. . Figure shows that both theoretical models agree with the simulations qualitatively, with an overestimation of the microgel size at low ionization similarly to the observations in Figs. and. On contrary to earlier cases, Langevin model is generally larger than the Gaussian one, which leads to a closer agreement between the Gaussian model and the simulations.