The intensity of an odour can be quantified by determining what is the response of an odour sensing device to that odour, then transforming the response data to an odour intensity value based on transformation data relating to a set of selected reference odours. The transformation data includes organoleptic data indicating how odour intensity values assigned to the set of reference odours by a sensory panel depend upon the concentration of the reference compounds and includes data indicating how the response data of the odour sensing device when exposed to the set of reference odours depends upon the concentration of the reference compounds. The reference odours may be basic odours defining the dimensions of a multi-dimensional space in which odours can be defined.