A novel permeation apparatus was developed which can make the on-line measurements of both flux and permeate composition. The development of the apparatus was motivated by the desire to measure flux transients, directly, rapidly and precisely. The apparatus was improved by solving the problems associated with the drawbacks in the continuous-flow technique. The transient measurements also allow the diffusion coefficient and concentration of a permeant in membrane to be rapidly determined. The apparatus performance was illustrated by conducting the pervaporations of water through a hydrophilic poly(vinyl alcohol) and a hydrophobic poly(dimethylsiloxane) membranes, respectively. A comparison of fluxes determined by the on-line measurement was made with those measured by the conventional method in which the permeate vapours were collected by liquid nitrogen for analysis. The diffusion coefficients and concentration of water in the membranes were determined from the flux transients and they were coincident with values in a literature. It was confirmed that fluxes with time could be measured precisely and rapidly with the apparatus.