The refractive index, density, and composition of fly ash from coal-fired boilers was investigated. The goals were to determine: (1) the interrelationship of refractive index and composition, and (2) the significance of ash properties on in-stack plume opacity. A survey was made of 14 ash samples representing a wide range of coals. Light absorption was measured using the Integrating Plate Method, which compares light absorption through a clean filter to that through a filter with a single layer of aerosol. Ony[sp] absorption is measured, while scattered light is integrated equally for both cases. This technique requires fine particles (volume absorbers) for easy interpretation of results. The technique was calibrated using an aerosol, methylene blue, with known absorption characteristics. The real part of the refractive index was measured by an oil immersion technique. The real refractive index and density were found to be highly correlated with composition with a multilinear regression equation. The absorbing refractive index was well correlated with ash carbon content. The modeling of in-stack opacity showed a weak dependence on ash optical properties for the range of ashes studied. The effect of the real part of the refractive index on opacity tends to be counterbalanced by particle density effects. Furthermore, except for very high carbon ashes, fly ash absorbs relatively little light.