A process for preparing (meth)acrolein by heterogeneously catalyzed gas phase partial oxidation of a precursor compound, in which the reaction gas mixture which comprises at least 3% by volume of precursor compound is passed through a fresh fixed catalyst bed whose active material is a multielement oxide which comprises the elements Mo, Fe and Bi and at least one of the elements Ni and Co, and the fixed catalyst bed is loaded with at least 40 l (STP)/l·h of precursor compound, and the precursor compound is converted to an extent of at least 90 mol %, wherein, over the first 8000 operating hours, during X=10 to 500 operating hours, the highest temperature of the reaction gas mixture in the fixed catalyst bed is 400 to 450° C. and, during the remaining 8000−X operating hours, is 300 to