The vibrational spectrum of the vapour phase above YI3 was measured by infrared spectroscopy. The spectrum revealed four strong bands, three of which could be assigned to the fundamantal infrared active vibrations of the YI3 monomer molecule. The observed frequencies agree very well with quantum-chemical calculations reported in literature. The presence of dimer molecules, the fraction of which is significant in the vapour according to literature, is also discussed in the context of assigning the fourth strong band in the experimental spectrum. Although its position matches well with a strong fundamental of the dimer, its intensity suggest a dimer fraction that is far outside the range for the other rare-earth triiodides.