Embodiments of our invention relate to processes for transitioning among polymerization catalyst systems including processes for transitioning among olefin polymerization reactions using Ziegler-Natta catalysts systems and chromium-based catalyst systems. Among embodiments contemplated are a method of transitioning from a first catalyst to a second catalyst in an olefin polymerization reactor, comprising: adding to the reactor a deactivating agent (DA) selected from one of carbon monoxide, carbon dioxide, or combinations thereof; adding to the reactor a cocatalyst adsorbing agent (CAA), comprising an inorganic oxide selected from one of silica, alumina or combinations thereof; wherein the first catalyst comprises at least one conventional Ziegler-Natta catalyst, and a cocatalyst, wherein the second catalyst comprises at least one chromium-based catalyst, wherein the reactor is a gas-phase, fluidized bed reactor, and wherein the CAA is substantially free of transition metals. In another embodiment a method of transitioning from a first catalyst to a second catalyst in an olefin polymerization reactor is contemplated, comprising: adding a transition aid agent (TAA) wherein the TAA is selected from one of alkoxylated amines, alkoxylated amides, or combinations thereof, wherein the first catalyst comprises at least one Ziegler-Natta catalyst comprising the catalyst, a cocatalyst and optionally a support, and the second catalyst comprises at least one chromium-based catalyst.