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Co-feeding water leads to a simultaneous attenuation of chain initiation and chain termination rates in HSSZ-13 catalyzed methanol-to-olefins (MTO) conversion. Density functional theory calculations and transient stoichiometric experiments support the plausibility of formaldehyde hydrolysis occurring over zeolitic Brønsted acid sites at MTO-relevant temperatures. A monotonic decrease in MTO chain initiation and termination rates, and a concurrent monotonic increase in total turnovers as a function of water co-feed partial pressure are consistent with the occurrence and mechanistic relevance of formaldehyde hydrolysis effected by co-fed water. Initiation/termination rates and total turnovers normalized by their corresponding values in the absence of water co-feeds at the same temperature show the expected trends as a function of reaction temperature, assuming equilibrium between formaldehyde and methanediol. These results underscore the implications of formaldehyde hydrolysis chemistry when assessing the mechanistic role of water in methanol-to-olefins conversion specifically, and deactivation mechanisms in zeolite-catalyzed hydrocarbon conversion processes more generally.