The dark clouds that were easily seen in small telescopes after the comet impacts were caused by small particles which were deposited in Jupiter's stratosphere. Observations from the Hubble Space Telescope and from ground-based instruments at visible and infrared wavelengths indicate that the mean radius of the particles is in the range 0.1 to 0.3 μm, and the total volume of particles is approximately the same as that for a 1-km diameter sphere. In the dark core regions of freshly-formed impacts, the particles are distributed over a large vertical extent, between about 1 mb and 200 mb or deeper. The diffuse outlying haze is confined to the high-altitude end of the range. Such a distribution probably reflects different methods of emplacement of the debris as a function of distance from the impact. The color of the particles, and their volatility as required to make waves visible, suggest an organic material as the main constituent. These relatively volatile materials are thought to have condensed onto more refractory grains after the plume material cooled, some 30 minutes or more after impact. The most refractory materials expected to condense from an evolving fireball are Al2O3, magnesium and iron silicates, and soot, depending on the C/O ratio. A silicate spectral feature was observed, confirming that cometary material was incorporated into the grains, although silicate grains make up only 10–20% of the particle volume.