The hypothesis was tested that beryllium metal particles have a uniformly thick surface coating of beryllium oxide and that smaller particles should have a higher oxide fraction by mass because they have a higher surface to volume ratio. The mass fraction of oxygen, physical density, and specific surface area were determined for size-fractionated samples of respirable beryllium metal particles. The largest particles analyzed (count median diameter 4.6 microns with geometric standard deviation 1.6) contained 7% +/- 1% beryllium oxide by mass; had a physical density of 1.90 +/- 0.02 g/cm3; and had a specific surface area of 4.0 +/- 0.3 m2/g. The smallest particles analyzed (count median diameter 0.4 micron with geometric standard deviation 1.8) contained 31% +/- 3% beryllium oxide by mass; had a physical density of 2.00 +/- 0.17 g/cm3; and had a specific surface area of 20.8 +/- 2.1 m2/g. These shifts in density and oxide content with size and surface area are consistent with a beryllium metal core of density 1.84 +/- 0.02 g/cm3 (1.848 g/cm3 is theoretical); a beryllium oxide layer of density of 2.53 +/- 0.16 g/cm3 (3.025 g/cm3 is the perfect crystalline density); and an oxide layer thickness of 49 +/- 4 A for all particle sizes. These results indicate that the inhalation toxicity of beryllium metal particles may be similar to that of beryllium oxide formed at low temperatures.