Perfusion microcalorimetry is frequently applied to expose pharmaceutical solids to specific relative vapour pressures (RVP) of organic liquids or water. The aim was to set up a general procedure to check the performance of a vapour pressure control device used to create a specific RVP. This was achieved by identifying and checking the significant factors in the creation of the RVP inside the reaction vessel. The flow switch valve was tested by volume flow measurements whereas the saturation of the wet line was verified in a calorimetric experiment employing methanol as model solvent. The results of the measurements were used to calculate the RVP inside the reaction vessel. The method developed is suitable to test the vapour pressure control device. It can be applied for any solvent. (C) 2009 Elsevier B.V. All rights reserved.