Photogeochemical reactions are described by the same principles used to describe photochemical reactions in general, and may be classified similarly:
# Photosynthesis: in the most general sense, photosynthesis refers to any light-activated reaction for which the change in free energy (ΔG) is positive for the reaction itself (without considering the presence of a catalyst or light).  The products have higher energy than the reactants, and therefore the reaction is thermodynamically unfavorable, except through the action of light in conjunction with a catalyst. Examples of photosynthetic reactions include the splitting of water to form H and O, the reaction of CO and water to form O and reduced carbon compounds such as methanol and methane, and the reaction of N with water to yield NH and O.
# Photocatalysis: this refers to reactions that are accelerated by the presence of a catalyst (the light itself is not the catalyst as may be erroneously implied). The overall reaction has a negative change in free energy, and is therefore thermodynamically favored. Examples of photocatalytic reactions include the reaction of organic compounds with O to form CO and water, and the reaction of organic compounds with water to give H and CO.
# Uncatalyzed photoreactions: photoinduced or photoactivated reactions proceed via the action of light alone. For example, photodegradation of organic compounds often proceeds without a catalyst if the reactants themselves absorb light.