Hypophosphorous acid (HPA), or phosphinic acid, is a phosphorus oxyacid and a powerful reducing agent with molecular formula HPO. It is a colorless low-melting compound, which is soluble in water, dioxane  
and alcohols.  The formula for this acid is generally written HPO, but a more descriptive presentation is HOP(O)H, which highlights its monoprotic character.  Salts derived from this acid are called hypophosphites.
HOP(O)H exists in equilibrium with the minor tautomer HP(OH).  Sometimes the minor tautomer is called hypophosphorous acid and the major tautomer is called phosphinic acid.