Milk powder deposits in drying devices can be a problem because of their tendency to self-heat. Smouldering milk powder becomes a source for ignition or even dust explosion. To this end, the exothermic reactivities of fresh whole and skim milk powders have been studied. This investigation examined the effect of thermal ageing and composition on the reactivities of milk powders. The kinetic parameters were found to change with ageing temperature for both whole and skim milk powders. The tendency of milk powder to ignite was found to increase with increasing fat content.