The ageing behaviour of a pyrotechnic mixture of magnesium and potassium nitrate has been followed at 50degreesC and 65% relative humidity by isothermal heat flow calorimetry. Measurements have been carried out with samples in air and in an inert atmosphere. The main reaction product was found to be magnesium hydroxide. This has been determined quantitatively by thermogravimetry and the amount formed correlated with the measured cumulative heat of ageing. The results have been compared with those obtained for magnesium powder studied under the same conditions. In addition the influence of the ageing process on the pyrotechnic reaction has been studied by high temperature differential scanning calorimetry under ignition conditions and modulated temperature differential scanning calorimetry. (C) 2003 Published by Elsevier Science B.V.