GaN films grown on (1 1 1) Si substrate by means of low pressure MOCVD technique in a horizontal flow quartz reactor are characterized by different thin layer analysis methods. The polycrystalline hexagonal structure of the GaN layers has been checked by means of grazing incidence X-ray diffractometry and IR spectroscopy. Cathodoluminescence (CL) spectra and their time kinetics are studied. The mean decay time of the 3,44 eV UV bound exciton transition is below I ns, whereas the 3.26 eV violet band shows a slow hyperbolical decay over about I ps. A third yellow band appears at 2.12 eV due to transitions via localized states.