Zinc-blende ZnSe powder, with particles containing nanometric grains, was prepared usingthe mechanical alloying technique, from an equiatomic mixture of elemental Zn and Sepowders. An important interfacial component was observed in the as-milled powder,which affects its thermal and optical properties. In order to obtain a high-qualityzinc-blende ZnSe powder an annealing process was applied. The structural properties ofboth as-milled and annealed powders were characterized by an x-ray diffractiontechnique. The effects of defect centers on the optical band gap of both as-milledand annealed zinc-blende ZnSe powders were studied through optical absorbancemeasurements. The results showed a substantial broadening and a shift of the band gapenergy to lower values in the as-milled powder, while for the annealed sample noeffects were observed. For the annealed sample, the measured band gap energyvalue is in good agreement with those reported in the literature. A sequence ofphotoacoustic absorption measurements of the as-milled ZnSe sample showed that theabsorbed energy promotes a structural relaxation. This relaxation causes a change inthe heat transfer mechanism from thermal diffusion to thermoelastic bendingand a reduction in the thermal diffusivity values. Similar measurements of theannealed nanostructured ZnSe sample showed the thermal diffusivity value to have ahigh degree of stability, and to be in good agreement with values reported in theliterature.