The common phosphorus oxoanion with P in the +5 oxidation state is the orthophosphate ion, PO₄^3-. This may polymerize to give longer chain molecules, such as diphosphate (or pyrophosphate), and also other polyphosphates.

orthophosphate: PO43-	diphosphate: P2O74-

PO₄^3- is less oxidizing than NO₃^-.

Phosphorus oxoanions with P in other oxidation states also exist.

hypophosphite: H2PO2-, P(I)	phosphite: HPO32-,P(III)	hypophosphate: P2O64-, P(IV)

Phosphorus Oxo-Acids

Phosphorus oxo-acids exist with formulae H₃PO₂, H₃PO₃ and H₃PO₄. These are the fully hydrogenated forms of the hypophosphite, phosphite and phosphate ions respectively. Despite having different numbers of hydroxy groups, the pK_a of each of these acids is the same. This can be explained empirically by Pauling's Rule:

For the oxo-acid E(OH)_n(O)_m, the pK_a = 8-5m.

The formulae for these acids can be written (with E=P) as (H)₂P(OH)(O), (H)P(OH)₂(O), and P(OH)₃(O) respectively, and so it can be seen that m in each case is 1 (there is only one P=O feature), and so the pK_a for all of these acids is 3.