Permeation fluxes of water and acetic acid from water-acetic acid mixtures were independently monitored as a function of time in differential permeation with dual detection. Two successive permeation waves were detected for water permeation and only one for acetic acid permeation in the transient regime. The onset of the second permeation wave for water coincides with that of acetic acid permeate whatever the condition. The time delay for the appearance of these waves decreases with increasing temperature and water content in the binary mixtures. The mean diffusion coefficient for different permeation waves and their activation energy were calculated from the permeation rates in the transient regime. The results strongly indicated a coupling of acetic acid flux with the second step of water permeation which probably results from the permeation of water-acetic acid aggregates.