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Some difficulty has been experienced with boric oxide powder for use on metal fires due to moisture adsorption and caking when stored in extinguishers. A series of experiments has been conducted on boric oxide powder samples of different particle size containing various amounts of flow additives, in order to investigate their moisture absorption and caking characteristics and fire performance. It was found that both the rate of moisture absorption and the crushing strength decreased with an increase in particle size and, to a lesser degree, with an increase in flow additive. Fire tests showed that the powder was more likely to form a coherent molten film if the stearate content was low and the particle size large. In view of these results it would appear that the optimum powder would have a larger overall particle size than that used at present, and would contain a maximum of 1 per cent stearate. This powder could conveniently be commercial grade boric oxide ground with 1 per cent magnesium stearate for only as long as required for coating the particles.