Transient permeation of oxygen, nitrogen, and carbon dioxide through certain cellulose acetate (CA) dense membranes made from solutions in dioxane and dioxane-added ethanol were studied by the flow method. In order to explain the overshoots for transient permeation rates in the latter case, a new model is proposed. It postulates a time-dependence solubility coefficient, in the form of a series of exponential terms, which is combined in Fick’s second law and solved. The model fits well the obtained permeation kinetics, leading to values of the diffusion, and permeation coefficients, and the structure change-related time. The overshoots are explained by the formation of unstable structure from dioxane-ethanol solvent mixtures, a memory effect of aggregated chains, which undergoes consolidation upon gas penetration. The stable state which results from the consolidation process is the same as that of membranes obtained from pure dioxane solutions.