The study of the rates of enzyme-catalyzed reactions. Rates of reaction are usually measured by using the purified enzyme in vitro with the substrate and then observing the formation of the product or disappearance of the substrate.

As the concentration of the substrate is increased the rate of reaction increases proportionally up to a certain point, after which any further increase in substrate concentration no longer increases the reaction rate. At this point, all active sites of the enzyme are saturated with substrate; any further increase in the rate of reaction will occur only if more enzyme is added.

Reaction rates are also affected by the presence of inhibitors, temperature and pH.