The time on stream behaviour of the selective (amm)oxidation of propene and propane over an active iron antimony oxide catalyst showed that the activity and selectivity of these catalysts decreased significantly with time on stream. Also gamma-Bi2MoO6 shows a strong time on stream behaviour. It is shown that the selectivity for acrolein increases strongly with time on stream for the bismuth molybdate catalyst. The activity, however, decreases strongly. The catalytic performance of iron antimony oxide catalyst can be improved by operating the partial oxidation/ammoxidation in a cyclic mode, i.e. alternating the feed from reducing (hydrocarbon/air(/ammonia)) to oxidising (air) atmospheres. It is shown that the re-oxidation of the catalyst can be performed at the same temperature as the reaction temperature. The original oxidation state is, however, not fully regained. Elevated temperatures or prolonged oxidation times are required to regain the original oxidation state of the catalyst.