H₃PO₃ on heating dissociates into orthophosphoric acid(H₃PO₄) and phosphine (PH₃). Since, the oxidation states of orthophosphorous acid (H₃PO₃), orthophosphoric acid (H₃PO₄) and phosphine (PH₃) are +3,+5 and -3 respectively, this reaction is known as a disproportionate reaction (since the oxidation states of one of the product are positive and the other is negative). The reaction is as follows.