GaN/AlN quantum dots grown by molecular beam epitaxy either on silicon (111), sapphire or 6H-SiC (0001) substrate have been investigated using photoluminescence, cathodoluminescence, Fourier transform infrared spectroscopy and attenuated total reflection spectroscopy. In-plane polarized intraband absorption is observed at energies of similar to 150 and 310 meV in GaN dots with 22 and 7.5 nm base diameter, respectively. For bigger dots, we observe three interlevel absorptions with a polarization component along the c-axis at energies ranging from 0.52 to 0.97 eV. Based on a simple 2D modelling of the confinement energies we show that the resonant absorptions involve conduction-band interlevel transitions between the electron ground state and states with one or two nodes along the c-axis and that in large dots, the internal field governs the transition energies. (C) 2002 Elsevier Science B.V. All rights reserved.