This paper presents an analysis of the influence of optical constants and particle size on the radiative properties and heat transfer involving ash clouds and deposits. Previous reported studies are mostly based on grey values of complex refractive index, which fails to reflect the true nature of the ash cloud and ash deposits. The analysis indicates that for radiative heat transfer calculations, information on size distribution of the ash cloud and wall emittance are more important than the radiative properties of ash cloud. For prediction of emittance of an opaque particulate ash deposit, careful estimation of absorption index is more important than the real index, which may be taken to be constant at 1.5, unless the deposit material is weakly absorbing and composed of fine particles. In such cases knowledge of spectral values of both real and absorption index appears to be necessary.