We propose a calorimetric method based on a time-dependent extension of the van't Hoff model of a depressing of the freezing temperature to investigate the slow permeation kinetics of a drug through the bilayers of a lipid vesicle, In particular we investigated the transfer of a drug from water to void unilamellar or multilamellar vesicles, performing the experiments at temperatures above and below the main transition (melting) of the hydrocarbon chains of the lipids. The results show a transient splitting of the peak associated to the main transition that disappears when the permeation kinetics reaches a steady-state condition, The phenomenon has been described by a model based on the experimental findings.