The molecule displays P(═O)H to P–OH tautomerism similar to that of phosphorous acid; the P(═O) form is strongly favoured.
HPA is usually supplied as a 50% aqueous solution and heating at low temperatures (up to about 90°C) prompts it to react with water to form phosphorous acid and hydrogen gas.
: HPO + HO → HPO + H
Heating above 110°C causes hypophosphorous acid to undergo disproportionation to give phosphorous acid and phosphine.
: 3 HPO → 2 HPO + PH