Gas phase decomposition of hydrogen phosphite H 2 PO 3 - and hypophosphite H 2 PO 2 - have been studied by using electrospray ionization-collision induced dissociat ion-tandem mass spectrometry (ESI-CID-MS/MS). Decom position of these ions involves the loss of H 2 molecules producing fragment ions PO 3 - and PO 2 - respectively (redox processes). The loss of H 2 O is a minor decomposition of H 2 PO 3 - .