The subject of the present invention is a process for preparing acrolein by dehydration of glycerol in the presence of a catalyst system based on iron phosphorous oxide containing, in addition, one or more elements chosen from alkali metals, alkaline-earth metals, AI, Si, B, Co, Cr, Ni, V, Zn, Zr, Sn, Sb, Ag, Cu, Nb, Mo, Y, Mn, Pt, Rh and the rare earths La, Ce, Sm. The process is preferably carried out in the gas phase in the presence of oxygen starting from aqueous solutions of glycerol. The process according to the invention makes it possible of obtain high acrolein selectivities.