We report on the results of an investigation of GaN layers by two-color spectroscopy in the visible and the near infrared (IR) (0.9-2 mum) or mid-infrared (6-18 mum). After primary hand-to-hand excitation, a secondary beam is used with photon energy to matching ionization energy of levels in the gap. Photoluminescence spectra obtained with and without additional IR illumination are compared for different photon energies. The impact of IR radiation on the donor-related emission is discussed, and the involvement of deep mid-gap states is postulated. (C) 2002 Elsevier Science Ltd. All rights reserved.