AbstractThe gas‐phase behaviour of organophosphorus compounds such as diethylmethylphosphonate towards electron impact (EI) ionization occurring in a quadrupole ion‐trap instrument is investigated. If all ions observed in conventional EI mass spectra are detected, ion/molecule reactions are seen to occur leading to formation of protonated molecules and adduct ions such as 2M+·and [2M + H]+. The keto‐to‐enol‐like isomerization of the molecular M+·ion seems to favour proton transfer to neutral M species and a “self chemical ionization” ion is observed with correspondin