The determination of food authenticity and the detection of adulteration are problems of increasing importance in the food industry. This is especially so for ‘value-added’ products, where the potential financial rewards for substitution with a cheaper ingredient are high. In this paper, the potential of infrared spectroscopy as a rapid analytical technique for the quantitative determination of adulterants in extra virgin olive oil is demonstrated. The method uses Fourier transform infrared spectroscopy, combined with attenuated total reflectance and partial least squares regression. Model systems comprising two types of ‘contaminant’ oil — refined olive and walnut — are investigated.