The reactivity of sulfur dioxide (SO2) molecules toward iron oxide cationic clusters (FemOn+) is studied by a homemade time-of-flight mass spectrometer coupled with a laser ablation/supersonic expansion cluster source and a fast flow reactor. The association products FemOnSO2+ can be observed for most of the clusters. The interesting result is that the cooperation effect of SO2 and water is in favor of the adsorption of gas phase water on specific scale iron oxide clusters (Fe2O2+ and Fe3O3+ ). The reactivity information obtained may be useful to investigate atmospheric heterogeneous chemistry of related systems. The reactivity of sulfur dioxide (SO2) molecules toward iron oxide cationic clusters (FemOn +) is studied by a homemade time-of-flight mass spectrometer coupled with a laser ablation / supersonic expansion cluster source and a fast flow reactor. The association products FemOnSO2 + can be observed for most of the clusters. The interesting result is that the cooperation effect of SO2 and water is in favor of the adsorption of gas phase water on specific scale iron oxide clusters (Fe2O2 + and Fe3O3 +). The reactivity information obtained may be useful to investigate atmospheric heterogeneous chemistry of related systems.