Stability research on gun propellants has been widely performed by microcalorimetry since the 1980s. TNO Prins Maurits Laboratory has already a broad experience since the early 1970s. In the past many studies were performed, to investigate the influence of oxygen, humidity etc. Less attention was paid to two other important aspects, namely the sample geometry and the filling degree during the microcalorimetric test. A statement in the old Dutch military literature presents the following "It is a well-known fact that the free surface influences the decomposition rate of the gun powder, i.e. unground propellant decomposes slowly in comparison to ground propellant". This is the same for all types of propellant (Amsterdam, February 1924), which implies research on this topic, related to the stability prediction measured by microcalorimetry is important. Since the decomposition of nitrocellulose is influenced by the amount of oxygen and the surface area, the best way to investigate the stability of gun propellants is to measure it 'ammunition like'. This means that a combination of the propellant grain size and the filling degree of the ammunition should be used for investigation. For small caliber ammunition the filling degree is close to 95%, and for large caliber bag ammunition (e.g. 155 nun) it is around 60%. As a result on these important aspects, TNO has developed five different sizes of sample vessels, to investigate the propellant grains in the most 'ammunition like' condition. In this paper, an overview is given of the influences of relative humidity, different grain sizes and available oxygen. The available oxygen is adjusted by changing the oxygen content of the air or by changing the filling degree. (C) 2003 Elsevier Science B.V. All rights reserved.