The effect of doping NiMoO, with increasing Cs concentrations (1, 3 and 6%), on the crystalline structure, chemical nature and catalytic behaviour for the oxidative dehydrogenation of butane, was studied. The atomic ratio Mo/Ni = 1 and the cesium concentration were checked by several techniques (X-ray photoelectron spectroscopy, atomic absorption, inductively coupled plasma spectroscopy). The structural study of the (Y-and P-phases was carried out by high temperature X-ray diffraction and showed that Cs does not affect the molybdate structure at room temperature. However it makes the transition from o-to P-phase by heating difficult. Cs remains on the catalyst surface blocking pores and reducing BET surface area. The acid and basic natures of the catalysts were studied by NH, and CO, temperature programmed desorption, respectively. The formation of nitrogenated compounds during the NH,-TPD was observed. CO,-TPD shows an increase in the catalyst basicity with Cs doping, but overdoping was observed for the 6% Cs-NiMoO, catalyst. The catalytic tests showed a decrease of activity with the Cs content. In both the ct-and P-phases the selectivity to dehydrogenation products increases with the Cs contents, but tends to decrease for the 6% Cs-NiMoO,, due to the overdoping effect. This effect in the a-phase and the P-phase stabilisation in the catalyst with higher Cs contents, contributes to an increase of the activity normalised to surface area.