Nuclear reactor fuel pellets of PuO2 and UO2 are fabricated within safety enclosures at Hanford Engineering Development Laboratory, Richland, Washington. Samples of the aerosols coincidentally formed during a plutonium oxide and uranium oxide powder mixing operation and during the centerless grinding of mixed oxide reactor fuel pellets were taken from within the safety enclosures. A small seven-stage cascade impactor was used to determine the aerodynamic size distribution and concentration and a spiral-duct aerosol centrifuge, the Lovelace Aerosol Particle Separator (LAPS) was used to study the characteristics of the particles with respect to aerodynamic equivalent size. Alpha spectroscopy showed that up to 11% of the alpha activity of the aerosols was associated with 241Am. In the powder-mixing operation, the aerosol size distributions had activity median aerodynamic diameters (AMAD) equal to 1.9 ± 0.4 (S.D.) μm and geometric standard deviations (sigmag) of 1.59±0.08; alpha activity concentrations were best expressed as log-normally distributed with median of 45 nCi/l and geometric standard deviation of 1.8. For the centerless grinding the size distribution had AMAD values equal to 2.3±0.3 (S.D.) μm and geometric standard deviations (sigmag) of 1.6±0.1; alpha activity concentrations were much higher with a median of 7 μCi/l and geometric standard deviation of 1.9. The aerosol particles exhibited significant electrostatic charge. In vitro measurements in a lung fluid simulant showed the aerosol particles formed during powder mixing were more soluble than those formed during centerless grinding. Although relatively insoluble, both types of particles demonstrated higher solubility than reported for laboratory aerosols of high temperature treated 239PuO2.