Alkalinity is a chemical measure of water's ability to neutralize acids. Alkalinity is also a measure of the buffering capacity of water, or its ability to resist changes in pH after the addition of acids. The alkalinity of natural waters is primarily due to the presence of weak acid salts, although strong bases may also contribute in extreme environments, e.g. (–OH).

The factor affecting the corrosion and sedimentation of water is alkalinity. For example, the alkalinity of water used in steam boilers must be high enough to prevent corrosion of the boiler walls.