Raman spectroscopic measurements on synthetic air clathrates, prepared at different pressures and temperatures, are presented. The gas fractionation for clathrates formed at -2 and -20 degrees C is determined from the integrated vibron mode intensities of nitrogen and oxygen molecules. An increasing fractionation in favour of oxygen with increasing pressure is found. An asymmetric peak shape for the O-2 vibron mode was observed in air clathrates prepared at 300 bar and -2 degrees C. The asymmetry of the N-2 vibron mode in the same sample is much less pronounced.