Bulk and aerosol fly ash samples from the MERC 18''atmospheric fluidized bed combustor have been characterized by three analytical techniques: X-ray fluorescence spectrometry (XRF) for elemental analysis, x-ray powder diffraction (XRD) for mineral identification and scanning electron microscopy/x-ray emission (SEM) for regional and surface analysis. The combustion tests were carried out on Montana Rosebud sub-bituminous coal with limestone added as a sulfur sorbent. Bulk samples were removed from the bed and particle collecting devices. Aerosol sampling from the effluent stream was conducted by the Lovelace Inhalation Toxicology Research Institute. The results of these analyses indicate that (1) the fluidized-bed fly ash is not composed of spherical, homogeneous particles, but irregularly shaped particles; (2) particle size distributions of the aerosols may exhibit multimodal behavior; (3) excess calcium from the bed may react with the aluminosilicate fly ash to form crystalline calcium aluminosilicates; and (4) calcium sulfate predominates in the small particles.