The influence of the fuel to oxidant ratio on the ageing behaviour of the magnesium-strontium nitrate pyrotechnic system in the presence of water vapour has been investigated by isothermal microcalorimetry in conjunction with chemical and thermal analysis measurements. The major products of ageing were identified as magnesium hydroxide and strontium nitrite. Measurement of the ratio of the amount of magnesium hydroxide formed to that of strontium nitrate reacted has shown that for compositions containing from 10 to 60% magnesium, a major component of the ageing process could be represented by the equation 3Mg + Sr(NO3)(2) + 4H(2)O --> 3Mg(OH)(2) + Sr(NO2)(2) + H-2 DSC studies have confirmed that the formation of strontium nitrite during ageing has a significant influence on the high temperature pyrotechnic reaction between magnesium and strontium nitrate. (C) 2004 Elsevier B.V. All rights reserved.