An extensive experimental study has been made of the extinguishment of a small n-heptane diffusion flame (14,7 cm diameter pan) by five common dry-chemical powders-potassium bicarbonate, sodium bicarbonate, potassium chloride, monoammonium phosphate, and Monnex. The fire extinguishing effectiveness (reciprocal of the observed minimum extinction concentration) of each drychemical substance, other than that due to chemical specificity, is fully explained on the basis of particle size and surface area. An important finding of study is a discontinuity in the extinguishing effectiveness of a powder as a function of particle size. This occurs for each substance at a unique particle diameter above which there is a dramatic five- to eight-fold decrease in effectiveness. For a given substance, all powders with particle sizes below the limiting value exhibit the same maximum effectiveness.