The aging of pyrotechnic mixtures Zr-PbCrO₄ was proved experimentally through its action on the burning rate and on some physical properties of the reactive medium. Complementary experiments were performed on freshly prepared mixtures in order to determine the nature of the process. It was concluded that aging is due to the formation of an oxygen-rich layer on the surface of the zirconium grains; the presence of this layer brings about decreases in the solid-solid contact and in the thermal conductivity of the medium.