A new vacuum permeation cell is described whose development was motivated by the desire to measure permeation transients directly, rapidly and precisely. The new cell also allows direct, rapid and precise measurement to be made of the response time of the measurement system itself, enabling measurement corrections to be easily assessed and applied. The transient measurements allow diffusion coefficient, flux, permeant concentration in the membrane and solubility coefficient to be rapidly determined. The cell behaviour is illustrated using the rapid diffusion of water through poly(dimethylsiloxane), a stringent performance indicator. Significant differences between these measurements and measurements previously published are briefly mentioned.