Composite membranes containing molecular sieve particles embedded in a polyimide matrix are promising due to their increased permeability and high selectivity in gas separation processes. Determination of permeability of dense membranes is time-consuming and the resulting values are loaded with experimental errors. The impact of uncertainty in various quantities on the reliability of the permeability values measured by the constant volume/ variable pressure method was analyzed. The total uncertainty of the measurements on polyimide/Silicalite-1 membranes is 7–13 %, the errors in membrane thickness and permeation coefficient being the main contributing factors.