Orthocarbonic acid, carbon hydroxide or methanetetrol is the name given to a hypothetical compound with the chemical formula .mw-parser-output .template-chem2-su{display:inline-block;font-size:80%;line-height:1;vertical-align:-0.35em}.mw-parser-output .template-chem2-su>span{display:block;text-align:left}.mw-parser-output sub.template-chem2-sub{font-size:80%;vertical-align:-0.35em}.mw-parser-output sup.template-chem2-sup{font-size:80%;vertical-align:0.65em}H4CO4 or C(OH)4. Its molecular structure consists of a single carbon atom bonded to four hydroxy groups. It would be therefore a fourfold alcohol. In theory it could lose four protons to give the hypothetical oxocarbon anion orthocarbonate CO4−4, and is therefore considered an oxoacid of carbon.

Orthocarbonic acid is highly unstable. Calculations show that it decomposes spontaneously into carbonic acid and water:

Orthocarbonic acid is one of the group of ortho acids that have the general structure of RC(OH)3.The term ortho acid is also used to refer to the most hydroxylated acid in a set of oxoacids.

Researchers predict that orthocarbonic acid is stable at high pressure; hence it may form in the interior of the ice giant planets Uranus and Neptune, where water and methane are common.

By loss of one through four protons, orthocarbonic acid could yield four anions: H3CO−4, H2CO2−4, HCO3−4, and CO4−4.

Numerous salts of fully deprotonated CO4−4, such as Ca2CO4 or Sr2CO4, have been synthesized under high pressure conditions and structurally characterized by X-ray diffraction. Strontium orthocarbonate, Sr2CO4, is stable at atmospheric pressure. Orthocarbonate is tetrahedral in shape, and is isoelectronic to orthonitrate. The C-O distance is 1.41 A. Sr3[CO4]O is an oxide orthocarbonate, also stable at atmospheric pressure.

The tetravalent moiety CO4 is found in stable organic compounds; they are formally esters of orthocarbonic acid, and therefore are called orthocarbonates. For example, tetraethoxymethane can be prepared by the reaction between chloropicrin and sodium ethoxide in ethanol. Polyorthocarbonates are stable polymers that might have applications in absorbing organic solvents in waste treatment processes, or in dental restorative materials. The explosive trinitroethylorthocarbonate possesses an orthocarbonate core.