Decomposition is the breakdown or degradation of complex organic substances into simpler substances by chemical or biological processes.

Organic material can be degraded in a series of reactions either aerobically, in the presence of oxygen, to carbon dioxide and water, or anaerobically without the presence of oxygen to yield methane and carbon dioxide.

Aerobic decomposition of an organic compound:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
Anaerobic decomposition of an organic compound:
C6H12O6 → 3 CH4 + 3 CO2

The breakdown of organic matter is also referred to as oxidation where oxygen is added to an element or compound.
In the environment, organic matter can be oxidized by bacteria, yielding energy to the organism. An oxidation reaction is always accompanied by a reduction reaction and vice versa. A reduction is the removal of oxygen from an element or compound. For example, sulphur compounds (SO4-2) are under anoxic conditions reduced to the odour-producing compound hydrogen sulphide (H2S) by simultaneous oxidation of organic matter.