This contribution presents several new experimental results provided by the techniques of X-ray diffraction at grazing incidence, photoluminescence at 6 K, C-Q-V and charge version of deep level transient spectroscopy on porous silicon (thickness similar to 1 mu m)/crystalline silicon structures prepared by the standard electrochemical etching process. An existence of new XRGI distinct reflection at 2 theta = 38.5 degrees, coming from the polycrystalline structure of the porous silicon layer, has been confirmed. For the first time we revealed a reversible metastability feature of two groups of gap states with the thermal activation energies of similar to 0.53 and similar to 0.70 eV, respectively. The metastable properties are related to the changes of both the water and hydrogen contents in the porous silicon layer induced by a low temperature annealing process.