A simple method suitable for the first estimation of the presence of volatile (inert) impurities in physical vapour transport systems is reported. The method, which is based on the use of a one-dimensional steady-state model, allows to calculate the overall pressure in the system when experimental and calculated mass transport rates are matched. When applied to the physical vapour transport of polycrystalline urotropine, the method proved suitable for evidencing the presence of volatile impurities whose partial pressures can be estimated to exceed the partial pressure of urotropine vapours of about one order of magnitude.