This paper presents the employment of a method, which combines in-situ IR spectrometry, on-line mass spectrometry, comparative (moisture-free vs moisture-containing) experiments, and isotope exchange, to reveal the deactivation mechanism of fluorine-promoted Pt/KL zeolite-type naphtha reforming catalysts. Fluorine leaves the catalyst in the forms of COF2 and HF as hydrocarbon feedstock and trace amount of H2O coexist. (C) 2011 Elsevier B.V. All rights reserved.