The commonly used spectral dependences for the emissivity of aluminum oxide particles are investigated. Theoretical considerations from Mie theory and the optical properties of alumina give rise to inverse wavelength and (-2) dependences when particles fall within the Rayleigh scattering regime. Analysis of particles of an arbitrary size (i.e. outside the Rayleigh regime) demonstrates that no general relationship exists between the wavelength and the spectral emissivity of these particles contrary to what has been routinely applied in the literature. This result was attributed to the dependence of both the spectral dependence and magnitude of the complex refractive index on the spectral emissivity. The significance of these results to applications (e.g. aluminized propellants) is discussed.