Abstract: The negative ion electron impact mass spectra(NIEJ-MS)and their cleavage in somephosphonothionates were discussed on the basis of low resolution mass spctral and meta-stable ion analysis. It is found that the property and the position of the substituentshave a marked influence on the isomeric rearrangement of the phosphonothionate.

Key words: phosphonothionate, negative ion electron impact mass spectra, isomeric rearangement