Persistent photoconductivity (PPC) has been investigated in detail in a Zn0.3Cd0.7Se mixed crystal. Two different temperature conductivity states have been observed. Relaxation of stored charge carriers, which contribute to PPC, has been studied at different conditions. We find that the decay of PPC follows the stretched-exponential function that is usually observed in a wide class of disordered materials. At T<220 K, the relaxation time increases with increase of temperature. At a constant temperature, the relaxation time increases with increase of excitation photon dose, which is a consequence of the presence of the two different conductivity states. However, at a constant temperature, the decay exponent is excitation-photon-dose independent, while the characteristic decay time constant depends on excitation photon dose. The PPC observed here thus exhibits characteristic phenomena of disordered systems, which suggests that the random local-potential fluctuations, which arise from the compositional fluctuations, are responsible for PPC. PPC-decay behavior is also analyzed for some of the previously published data on other materials. We find that the stretched-exponential function describes the PPC decay in various materials at low temperatures.