| Acid | Formula | Formal oxidation state of phosphorous |
| Pyrophosphorous acid | H4P2O5 | +3 |
| Pyrophosphoric acid | H4P2O7 | +5 |
| Orthophosphorous | H3PO3 | +3 |
| Hypophosphoric acid | H4P2O6 | +4 |
Both pyrophosphorous and orthophosphorous acid have +3 formal oxidation state