An approximate heuristic expression for the particle self-shielding factor for a set of purely absorbing spheres of radius r and volume fraction V well mixed with another set of non-absorbing spheres has been derived. The resulting expression has been experimentally verified using transmission data at several incident neutron energies for a plate-type sample containing a mixture of aluminum and boron-carbide spheres with nominal diameters 85 ± 15µ. The boron-carbide spheres occupied about 37% of the sample volume. The transmission was measured at energies ranging from 0.03 to 1.2 ev using a crystal neutron spectrometer. Since, however, the sample contained boron-carbide spheres with a distribution of diameters, the experimental self-shielding factors are “average” values. It is shown, using an approximate model, that a plausible theoretical self-shielding factor is a volume weighted average of the self-shielding factors for the spheres of diameters, d1, d2, d3, … . The particle self-shielding factors derived by several other authors have also been compared with the present experimental results. The Hurwitz-Zweifel expression (1) gives quite bad agreement, though this is expected because of the high volume fraction of poison in the sample. The Burrus expression (2, 3) gives much better agreement though not as good as the expression derived in this paper.