Membranes based on cellulose acetate for reverse osmosis can possibly be applied to the so-called salinity process of energy generation and water desalinization. The requirements for membranes for these two different applications are a relatively high water flux and low salt permeability. In this article, we present the optimization of the composition of such membranes. We started by producing membranes with a patented casting solution with the following composition: 45.77 wt % dioxane, 17.61 wt % acetone, and 8.45 wt % acetic acid (solvents); 14.09 wt % methanol (nonsolvent); and 7.04 wt % cellulose diacetate and 7.04 wt % cellulose triacetate. The membranes produced with this solution were analyzed comparatively, with the membranes obtained by the introduction of modifications to the following parameters: the solvent mix, the nonsolvent mix, the proportion of cellulose diacetate and cellulose triacetate in the casting solution, and the addition of reinforcing cellulose fibers. The results led us to conclude that the best membrane formulation had the following composition: 45.77 wt % dioxane, 17.61 wt % acetone, and 8.45 wt % acetic acid (solvents); 4.22 wt % cellulose triacetate and 9.86 wt % cellulose diacetate (polymers); 14.09 wt % methanol (non-solvent); and 0.5 wt % cellulose fibers (with respect to the total polymer content).