The “odour unit number” used in applied flavour research and known by several different names, is neither a unit of absolute physical concentration of an odorant, nor is it a psychophysical measure for perceived odour intensity. The use of this concept has been shown to rest upon implicit assumptions which are contradictory in some respects to present psychophysical knowledge. A special consequence for the mixture threshold value of the defined relationship between odour unit numbers of components and that number of the mixture from which they are parts was shortly discussed. Finally, a few words were devoted to a psychophysical approach in the study of interaction phenomena in odorant mixtures.