During air raids on this country in 1940-1 it was observed that H.E, bombs detonating in or near a flour mill generally gave rise to fire so widespread that totally destroyed. This was ascribed to flame through the suspension of flour in air set up propagation by the detonation. In order to prevent this kind of fire a scheme of protection was adopted in which an inert dust was distributed throughout the mill in bags in such a way as to be raised with the flour in the event of disturbance and thus inhibit flame propagation. The dust selected for use consisted of a mixture of 90% powdered limestone and 10% sodium bicarbonate. the latter greatly enhancing the flame suppressing power of the former. In order that the inert dust might effectively fulfil its function, good dispensability was important and the definition, measurement and improvement of this have been studied in the present investigation. The presence of a soluble constituent in the mixture gave rise to the possibility of gross loss of dispersability on account of caking due to moisture and the incidence and prevention of this effect have been investigated. Conditions conducive to the formation of local concentrations of bicarbonate by segregation of the constituents tinder the influence of vibration have also been examined. As a result of these investigations, indications are given of the way in which the factors of inert dust composition and particle size may be best all round specification for use in flour mills. On principles the details of a scheme for a cork mill, in which the dust explosion hazard is greater still, is worked out.