We report an investigation, performed in the full composition range x=0–1, of the change in infrared reflectivity spectra of AlxGa1-xN layers deposited on 6H–SiC substrates. We have found two different transverse E1(TO) phonon frequencies that can be assigned to AlN-like and GaN-like modes. The composition dependences of these frequencies can be well approximated by linear functions and the oscillator strengths scale like the corresponding Al and Ga mole fractions, respectively. On a purely experimental basis, this establishes evidence of a two-mode behavior for this controversial alloy system. The frequencies of the impurity mode of Ga in AlN (622 cm-1) and of the impurity mode of Al in GaN (643 cm-1) were determined. © 1998 American Institute of Physics.