The mechanism of the so-called standard SCR reaction on an efficient HBEA zeolite catalyst modified with 0.25wt.% Fe (0.25Fe/HBEA) was elucidated. The studies were systematically performed by using DRIFTS, TPD, XANES and kinetic studies. SCR kinetics were examined between 175 and 275°C employing a gradient-free loop reactor. DRIFTS and NO x -TPD showed formation of Fe 3+ -NO species as well as their reaction with NH 3 . But, only minor adsorption and conversion of NO x on the zeolite substrate occurred. On the contrary, DRIFTS and NH 3 -TPD showed mainly adsorption of NH 3 on the zeolite, whereas XANES and kinetic examinations indicated NH 3 adsorption on the Fe 3+ sites as well. Moreover, the XANES studies evidenced deNO x on the iron component which is considered to be the major pathway in standard SCR. From our experimental investigations we suggest a dual site mechanism implying the adsorption and reaction of NO and NH 3 on neighbouring Fe 3+ sites. The uptake of NH 3 results in partial reduction of Fe 3+ sites which are finally recycled by O 2 . However, some contribution from a single site mechanism cannot be ruled out completely.Numerical modelling of the NH 3 -TPD pattern provides kinetic parameters of the adsorption and desorption of NH 3 on 0.25Fe/HBEA differentiating the specific adsorption sites of the substrate. As a result, the uptake and release of NH 3 on the zeolite is found to be much faster than that of SCR. In connection with a previous SSITKA study, this comparison suggests that NH 3 undergoes several adsorption/desorption cycles on the substrate before adsorbing and reacting on the Fe 3+ sites.