The influence of promoters, potassium and samarium, on molybdenum supported over MgO-gamma-Al2O3 catalyst has been investigated in the oxidative dehydrogenation of propane. The acidities of catalysts were determined by temperature-programmed desorption of NH3 and by decomposition of 2-propanol. The K-promoted catalyst showed the lower acidity followed by the Sm, whereas the unpromoted sample showed the highest acidity. The higher the acid character of the catalyst, the lower the selectivity to propene. Redox properties determined from EPR spectra change with the addition of the promoter. A parallelism between Mo6+ reducibility and catalytic activity was found.