Water is present in both conventional chemical transformations and conversion of renewable emerging feedstocks such as biomass and plastic waste. Solid acids and zeolites, in particular, have been one of the most common industrial heterogeneous catalysts for decades. Interestingly, the interactions of water with reaction intermediates and framework protons in Brønsted zeolites give rise to various catalytic effects. In the first part, the role of water in the acylation of a phenolic compound with a carboxylic acid representing the upgrading reaction of lignocellulosic biomass has been revealed. The bulky intermediate ester is an active acylating agent but diffuses slowly in MFI pores, which can be manipulated by co-feeding water. In the second part, the effect of water on the conversion of a non-polar hydrocarbon was studied. The presence of a sufficient concentration of water inside the zeolite pores enables the formation of hydrated hydronium ions, which enhances the cumene dealkylation due to entropic gain of the reaction transition state. Finally, the influence of water on the dehydration of a diol representing polar polymers in multilayer polymer films in the presence of a competitive polar aprotic solvent was investigated. The change in the adsorption states of the reaction intermediate and solvent molecules on framework Brønsted acid sites versus hydrated hydronium ions alleviates the inhibition effect of solvent and improves the dehydration of polyols.