The present contribution investigates shape effects and surface effects in the infrared spectra of pure (N(H)3) and mixed (NH3-CO2 and NH3-NH2D-NHD2-ND3) ammonia particles with sizes between about 1 and 50 nm. The particles investigated have been generated in a collisional cooling cell as aerosols at temperatures between 20 and 80 K. The contribution reveals that only the combination with a microscopic model leads to a comprehensive understanding of the various features observed in the experimental infrared spectra. As one of the major results, the corresponding exciton model explains why pronounced shape effects observed for pure particles only play a minor role in the case of mixed particles. (C) 2004 American Institute of Physics.