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• By measuring the dielectric properties of 2D confined water and analyzing the relaxation process, the permeation rate of water and the performance of alcohol-water separation can be predicted. • Cationic intercalation can modulate the viscosity of 2D confined water, further impacting water permeation. • Compared with other characterization, dielectric spectroscopy shows the potential for accurate prediction on water permeation without the need of subtracting the adsorbed water. • The prediction on separation performance by dielectric analysis shows universality for other 2D membranes. Attributed to the growing demands for biomass fuels, the alcohol-water separation membranes, particularly the two-dimensional (2D) lamellar structures with high permeability and selectivity have attracted increasing attention. However, owing to the limited tools available for investigating the mass transport in the 2D nanocapillaries, the development of separation membranes mainly relied on theoretical prediction and a summary of experimental performance. In this work, we present an in-operando dielectric study on the pristine and varied ion-crosslinked 2D membranes for alcohol-water separation to establish the relationship between the relaxation process and the separation performance of the membranes. We found that the dielectric relaxation process in the range of 10−2 to 102 Hz, which is related to the viscosity of water in 2D nanocapillaries, can act as an indicator of the separation performance of the membranes. Furthermore, the slowing down of the relaxation process due to the electric field around the cations indicates lower water permeance and separation factor. Compared to the previous characterizations on the surface hydrophilicity or the water absorption, monitoring the dielectric process directly offers insights into the interlayered molecular states and transport, and therefore opens a new avenue towards rational design of 2D selective membranes for alcohol-water separation. [ABSTRACT FROM AUTHOR]