Experimental evidence has shown that graphene oxide (GO) can be impermeable to liquids, vapors and gases, while it allows a fast permeation of water molecules. Theoretical studies to understand the filtration mechanisms come mostly from water desalination, while very few works have been dedicated to alcohol dehydration. In this work, we have investigated the molecular level mechanism underlying the alcohol/water separation inside GO membranes. A series of Molecular Dynamics and Grand-Canonical Monte Carlo simulations were carried out to probe the ethanol/water and methanol/water separation through GO membranes composed of multiple layered graphene-based films with different interlayer distance values and number of oxygen-containing functional groups. Our results show that the size exclusion and membrane affinities are not sufficient to explain the selectivity. Besides that, the favorable water molecular arrangement inside GO 2D-channels forming a robust H-bond network and the fast water permeation are crucial for an effective separation mechanism. In other words, the separation phenomenon is not only governed by membrane affinities (enthalpic mechanisms) but mainly by the geometry and size factors (entropic mechanisms). Our findings are consistent with the available experimental data and contribute to clarify important aspects of the separation behavior of confined alcohol/water in GO membranes.