A hydration reaction is a reaction that requires a water molecule. In the special case that the hydration reaction splits a molecule into two components, the process is known as hydrolysis. For example, when breaking a biopolymer such as a nucleic acid into its constituent monomers, a hydrolysis reaction replaces half of the polymer bond with a hydroxyl group (OH) taken from the water molecule and the other half with a single hydrogen atom (H).

The opposite reaction of hydration is called dehydration.