The optical properties of composite particles are calculated by means of the discrete dipole approximation. In particular, efficiency factors for extinction' scattering and absorption, asymmetry parameter, linear polarization, and phase function S-11 are obtained for particles in which (a) the individual components of different materials are distributed randomly within the particle body, and (b) the individual constituents are not distributed randomly over the particle volume, but are clumped together into larger compact blocks. Particles are assumed to have non-spherical forms (ellipsoids, cuboids, and an irregular shape U2015B10 are discussed in detail). Computational results are compared to those obtained for homogeneous particles of the same morphology using an effective medium theory (EMT). The greatest discrepancies between the use of the homogeneous and the inhomogeneous particles are found for polarization when particle size is larger than the wavelength of incident radiation. An important finding for S-11 is that the particles built from compact homogeneous blocks contribute more efficiently to backscattering than other analyzed particles, leading to a consistently lower asymmetry parameter. The homogeneous particles scatter least in the backscatter region, resulting in the highest asymmetry parameters of the three classes of particles. We find that the internal heterogeneities and surface structure have a comparable effect on the asymmetry parameter. It is shown that the efficiency factor for scattering is reduced when the real material configuration is ignored and the optical properties are modeled using the EMT; however, the use of the EMT and homogeneity assumption leads to significantly increased absorption. (c) 2006 Elsevier Ltd. All rights reserved.