Acrolein is prepared by a process of heterogeneous catalyzed partial gas phase oxidation, by passing a starting reaction gas mixture which comprises propene, molecular oxygen and at least one inert gas, in which the molecular oxygen to propene (O.sub.2:C.sub.3H.sub.6) mole ratio is .gtoreq.1, and carbon dioxide and saturated hydrocarbons together in a total amount of at most 15 mol %, at elevated temperature and at an hourly space velocity on the fixed catalyst bed of propene present in the starting reaction gas mixture of .gtoreq.120 l (STP)/lh through a fixed catalyst bed whose catalysts are comprised of annular unsupported catalysts whose active component is at least one multimetal oxide of formula I, Mo.sub.12W.sub.aCo.sub.bFe.sub.cBi.sub.dSi.sub.eK.sub.fO.sub.n (I) wherein a ranges from .gtoreq.1 to .ltoreq.3, b ranges from .gtoreq.3 to .ltoreq.8, c ranges from .gtoreq.1 to .ltoreq.4, d ranges from .gtoreq.0.5 to .ltoreq.1.5, e ranges from .gtoreq.0 to .ltoreq.10, and f ranges from .gtoreq.0 to .ltoreq.0.2 and n is a number which is determined by the valency and frequency of the elements in the composition of formula I other than oxygen, in such a way that the propene conversion in single pass is .gtoreq.90 mol % and the associated selectivity of acrolein formation is .gtoreq.80 mol %, wherein, in the multimetal oxide composition of formula I, the following mole ratios are: Co/Fe=from 2 to 4 and Co/Mo=from 0.3 to 0.7.