Diamond particles were employed to toughen zinc sulfide--an infrared transparent, but mechanically weak material in its pure state. The optical properties of pure ZnS were preserved in a composite when the diamond particles were well dispersed and, when compared to the wavelength, their sizes were sufficiently small. Careful control of the processing parameters was required to maintain a small grain size and to limit the phase transformation of ZnS to a non-cubic phase at high temperatures. Measurement of the reflectivity of this composite in the region of the lattice vibration spectrum was used to compare the measured electrodynamic properties with those predicted by two different theoretical models that assume different microstructural morphology. Good agreement was found with the Bruggemann model that assumes both components in the composite have the same type of interconnectedness.