Abstract A range of Cs-doped Fe/Al₂O₃ catalysts were prepared for the ammonia decomposition reaction. Through time on-line studies it was shown that at all loadings of Cs investigated the activity of the Fe/Al₂O₃ catalysts was enhanced, with the optimum Cs:Fe being ca. 1. Initially, the rate of NH₃ decomposition was low, typically < 10% equilibrium conversion (99.7%@500°C) recorded after 1 h. All catalysts exhibited an induction period (typically ca. 10 h) with the conversion reaching a high of 67% equilibrium conversion for Cs:Fe = 0.5 and 1. The highest rate of decomposition observed was attributed to the balance between increasing the concentration of Cs without blocking the active site. Analysis of H₂-TPR and XPS measurements indicated that Cs acts as an electronic promoter. Previously, Cs has been shown to act as a promoter for Ru, where Cs alters the electron density of the active site, thereby facilitating the recombination of N₂ which is considered the rate determining step. In addition, XRD and N₂ adsorption measurements suggest that with higher Cs loadings deactivation of the catalytic activity is due to a layer of CsOH that forms on the surface and blocks active sites. Graphic Abstract