The dynamics of retained and deactivating species in a SAPO-18 catalyst during the methanol-to-olefins reaction have been followed using a combination of ex-situ and in-situ techniques in differential and integral reactors.The retained species were analyzed using extraction, in-situ FTIR and in-situ UV-vis spectroscopies combined with online product analysis (gas chromatography and mass spectrometry). The composition of the extracted soluble species was determined using gas chromatography-mass spectrometry and that of the insoluble species using high-resolution mass spectrometry. We observe a decrease in the formation and degradation rates of retained species when co-feeding water, whereas the extent of the decreases is the same across the entire spectrum of retained molecules. This indicates that co-feeding water unselectively quenches the formation of active and deactivating species. At the same time, the catalyst has an extended lifetime when co-feeding water due to the diffusion of species (particularly olefins) out of the SAPO-18 crystals, and subsequent growth of heavy polycyclic aromatic structures that imply less deactivation. These conclusions can be extrapolated to other MTO catalysts with relatively similar pore topology such as SAPO-34 or SSZ-13 structures.