A process for long-term operation of a heterogeneously catalyzed partial gas phase oxidation of propene to acrolein, in which a reaction gas input mixture which comprises propene, molecular oxygen and at least one inert gas and comprises the molecular oxygen and the propene in a molar O 2: C 3 H 6 ratio of ≥ 1 is conducted through a fixed catalyst bed whose active material is at least one multimetal oxide comprising the elements Mo, Fe and Bi, with the proviso that - the fixed catalyst bed is arranged in two spatially successive temperature zones A, B, - both the temperature T A of temperature zone A and the temperature T B of temperature zone B are a temperature in the temperature range from 280 to 420°C, - the reaction gas input mixture flows through temperature zones A, B in the time sequence "first A" and "then B", temperature zone A extending up to a conversion C A of the propene present in the reaction gas input mixture in the range from 45 to 85 mol%, and the conversion of the propene increasing in temperature zone B to a value C B of ≥ 90 mol%, - in single pass of the reaction gas input mixture through the overall fixed catalyst bed, the selectivity of acrolein formation, based on propene converted, is ≥ 80 mol%, - the space velocity of propene present in the reaction gas input mixture on the fixed catalyst bed is ≥ 140 1 of propene/l of fixed catalyst bed.h, - the temperatures T A and T B on completion of fresh charging of the fixed catalyst bed are such that the difference ΔT BA = T B - T A > 0°C, - then, with increasing operating time, in order to counteract the reduction in the quality of the fixed catalyst bed, at least one of the two temperatures T A, T B is increased, wherein - the increasing of the at least one of the two temperatures T A, T B is undertaken such that the difference ΔT BA T B - T A increases with increasing operating time. - the fixed catalyst bed is arranged in two spatially successive temperature zones A, B, - both the temperature T A of temperature zone A and the temperature T B of temperature zone B are a temperature in the temperature range from 280 to 420°C, - the reaction gas input mixture flows through temperature zones A, B in the time sequence "first A" and "then B", temperature zone A extending up to a conversion C A of the propene present in the reaction gas input mixture in the range from 45 to 85 mol%, and the conversion of the propene increasing in temperature zone B to a value C B of ≥ 90 mol%, - in single pass of the reaction gas input mixture through the overall fixed catalyst bed, the selectivity of acrolein formation, based on propene converted, is ≥ 80 mol%, - the space velocity of propene present in the reaction gas input mixture on the fixed catalyst bed is ≥ 140 1 of propene/l of fixed catalyst bed.h, - the temperatures T A and T B on completion of fresh charging of the fixed catalyst bed are such that the difference ΔT BA = T B - T A > 0°C, - then, with increasing operating time, in order to counteract the reduction in the quality of the fixed catalyst bed, at least one of the two temperatures T A, T B is increased, wherein - the increasing of the at least one of the two temperatures T A, T B is undertaken such that the difference ΔT BA T B - T A increases with increasing operating time.