Many clay-bearing sedimentary stones such as Portland Brownstone will swell when exposed to water, and this can generate damaging stresses as differential strains evolve during a wetting cycle. Current swelling inhibitors, consisting of [alpha], [omega]-diaminoalkanes, can reduce swelling in Portland Brownstone up to 50%. In this study, through X-ray diffraction and swelling strain experiments, we demonstrate that the [alpha], [omega]-diaminoalkanes inhibit swelling by substituting for interlayer cations and partially hydrophobicizing the interlayer, then rehydrating on subsequent wetting cycles. We also introduce the copper (II) ethylenediamine complex as a potential treatment for swelling inhibition.