This paper shows the use of the instrumental neutron activation analysis (INAA) technique to calibrate Aethalometer instruments' response to iron oxides present in Saharan dust aerosol. The five samples selected for this calibration were collected with a seven-wavelength Aethalometer (model AE31) at the summit of Pico mountain (2225 m ASL) in the Azores Islands. These samples correspond to measurements taken between the 31 October and 5 November, 2001. Analysis of these samples by the INAA technique gave a total mass of 14.84 +/- 0.70 mu g (Fe). Correlation of the analytical results with the optical measurements allowed the determination of the elemental iron calibration constant, K-Fe (0.234 +/- 0.022 mu m(4) m(2) g(-1)), which can be used in the determination of iron concentrations from multi-wavelength Aethalometer measurements. As an example of this, we used Aethalometer measurements during the 2001 event to calculate hourly average dust iron, (C-Fe), concentrations, which range from 0.00 and 1.77 mu g (Fe) m(-3). (C) 2006 Elsevier Ltd. All rights reserved.