The effects of pore-level geometry on direction-averaged radiative properties of small heterogeneous particles encountered in solar thermochemical applications such as three-dimensionally ordered macro-porous (3DOM) cerium dioxide (ceria) particles are investigated in the UV, VIS and IR spectral ranges, typically 0.3–10 μm. The porous particles are modeled as three-dimensional arrays of interacting dipoles using the discrete dipole approximation (DDA). Reference solutions are obtained by directly solving Maxwell's equations. Validity ranges of the Lorenz–Mie theory to predict far-field radiative properties of quasi-homogeneous particles with the effective optical properties obtained using the volume-averaging theories (VAT) are established. The properties of interest are the spectral extinction, scattering, and absorption efficiency factors as well as the scattering phase function and the scattering asymmetry factor.