The relative importance of possible factors leading to the deactivation of a commercial SO emission reduction additive used in the fluid cracking process was evaluated individually. This additive was based on mixed oxides of Mg and Al and contained small amounts of Ce and V. The laboratory deactivation simulation was an extended hydrothermal treatment of the additive by steam with or without the presence of sources of possible poisons such as Si, V, and S. It was observed that a reduction of the area and sintering and migration of V from the catalyst to the additive did not affect the performance of the additive. However, migration of Si from a fresh catalyst or sulfate from an impregnated catalyst drastically reduced the SOx adsorption capacity of the additives. As the migration of Si was important mainly with fresh catalyst, which is present in the inventory of the unit in very small quantity compared to the large amount of equilibrium catalyst, the main factor leading to deactivation of the SOx additives in the industrial operation can be attributed to formation of stable sulfates.