The influence of doping with Al and some iron‐group metals on optical and electrical properties of GaN epitaxially grown from the vapor phase is reported. These impurities are the main inadvertent contaminants in GaN growth, and details on the transport of these elements to the deposition area during growth are discussed. Detailed studies are performed by the SIMS technique on impurity concentration in layers grown under different conditions of contamination. Variations in doping over the area of the grown wafers, as well as with depth into the layer, are investigated, and shown to be significant for Al and N2. These investigations are complemented by SEM cathodoluminescence topographs. Photoluminescence data strongly indicate the existence of a shallow bound exciton (binding energy 12 meV) to isoelectronic Al on Ga sites in GaN. No radiative states are observed from the iron‐group contaminants. These (notably Fe and Cr) cause deep states efficient in electrical compensation of the material, which is easily made highly resistive with Fe or Cr.