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A phosphate triester is a phosphoric ester in which three organic groups are attached through oxygen to one phosphorus center. A general connectivity formula isUnlike a neutral-acid phosphate monoester or phosphate diester, this representation contains no remaining P–OH group. The organic groups may nevertheless contain their own ionizable sites; “triester” does not guarantee that every molecule in the class is electrically neutral.

Trimethyl phosphate supplies a concrete example: $\mathrm{O}=\mathrm{P}(\mathrm{OC}{\mathrm{H}}_3{)}_3$ has formula ${\mathrm{C}}_3{\mathrm{H}}_9{\mathrm{O}}_4\mathrm{P}$. Its three methyl groups each contribute one carbon and three hydrogens, while the central unit and linking oxygens contribute one phosphorus and four oxygens.^[ref]

An original connectivity comparison replaces one $-\mathrm{OC}{\mathrm{H}}_3$ group by $-\mathrm{OH}$. The resulting fragment, $\mathrm{O}=\mathrm{P}(\mathrm{OC}{\mathrm{H}}_3{)}_2(\mathrm{OH})$, is a diester because only two organic groups remain attached through oxygen. Merely removing a proton from that O–H group changes charge without restoring a third ester linkage.

The suffix counts esterification at one phosphorus center. Three separate phosphate monoesters elsewhere on a large molecule do not constitute one phosphate triester. This local counting rule helps distinguish a change in connectivity from a change in pH, protonation or total phosphorus content.

A phosphoric ester is an ester of phosphoric acid: an organic group is joined through oxygen to phosphorus. Replacing one, two or three of the acid's O–H hydrogens by organic groups gives a phosphate monoester, phosphate diester or phosphate triester. A phosphoric anhydride instead contains an acid-derived linkage, such as P–O–P, rather than an additional P–O–C ester linkage.^[ref]

For a structural counting example, compareThe first has one organic O–P attachment; the second has two. Removing an O–H proton changes the charge but leaves that attachment count unchanged. Conversely, adding a second separate monoester group elsewhere on a large molecule does not turn the first phosphorus center into a diester.

This distinction matters in biochemical measurements. 4-Nitrophenyl phosphate is a phosphate monoester whose cleavage can be followed optically; the released inorganic phosphate is a different chemical species. An alkaline phosphatase assay detects a specified conversion under specified conditions, rather than counting all phosphorus-containing material indiscriminately.^[ref]