Safety analysis of sodium-cooled plutonium-fueled fast reactor plants must be concerned with the possibility of fires involving these materials. Design of an air cleaning system for such a facility requires basic data defining the aerosol characteristics of sodium and plutonium released during a fire. Size characteristics of the aersol produced during sodium and plutonium fires were determined for different atmospheres ranging from 20.8% oxygen, 79.2% nitrogen to 100% nitrogen. The aerosol produced by burning gram quantities of sodium was compared with that produced by a fire involving 600 lb of sodium. Sodium aerosol count median diameter ranged from 0.07 to 1.09µand was independent of oxygen concentration. Small and large scale fires produced an aerosol with comparable size characteristics. Plutonium aerosol count median diameter ranged from 0.02 to 0.09µand was also independent of oxygen concentration. When plutonium alloy was burned under reduced oxygen conditions, the fraction airborne ranged from 2.×10-7to 4.×10-6. Fires involving plutonium alloy and sodium together produced airborne plutonium-sodium ratios ranging from 0.34 to 0.008%.