The swelling and dissolution of cellulose are key parameters in the production of regenerated cellulose fibers. Since cotton is almost pure cellulose, it has been proposed that the recycling of cotton textiles may be accomplished through incorporating the cotton textiles into the production of regenerated cellulosic fibers. In this study, the supramolecular structure before and after pretreatment was characterized using solid-state carbon-13 nuclear magnetic resonance (13C NMR), and the findings related to the swelling of dissolving pulp and cotton were quantified with a fiber analyzer. The cotton and dissolving pulp samples were subjected to three different pretreatments: mild acid hydrolysis, acid hydrolysis in ethanol, and a hydrothermal treatment. The results showed that cotton was harder to swell than the dissolving pulp. This indicated that either waste cotton requires another type of activating pretreatment than those included in this study or very good solvents if it is to be included in the production of regenerated fibers.