Abstract The behavior of HZSM-5 zeolites under flowing 1-butene was studied directly in an IR spectrophotemeter with a diffusive reflection (DR) attachment at 398, 423 and 473 K. The spectroscopic data were analysed by the Schuster-Kubelka-Munk function. Thermogravimetric analysis confirmed the spectroscopic results. The gas leaving from the DR-IR cell was analysed by gas chromatography. During 1-butene adsorption, OH acid sites on the zeolite disappear, while hydrocarbon deposits, whose nature depends on the reaction temperature, are formed. After an initial period, during which deposit formation takes place, the zeolite still shows isomerizing activity, the selectivity depending on the reaction temperature.