The electron ionization and Brønsted acid chemical ionization mass spectra of the diethyl esters of succinic acid, methylmalonic acid, glutaric acid, ethylmalonic acid and dimethylmalonic acid have been determined. The major primary fragmentation reaction of the molecular ion in the electron ionization mass spectra is by loss of OC 2H 5 while the MH + ions fragment by loss of C 2H 5OH to form the same fragment ion. Using isotopic labelling (diethyl-d 5 esters) and metastable ion studies, it is shown that the [MOC 2H 5] + ions formed from diethyl succinate and diethyl glutarate have ethyl-cationated cyclic anhydride structures which fragment further by elimination of C 2H 4 to form the protonated anhydride. For the remaining esters the [MOC 2H 5] + ions have an acylium ion structure and fragment primarily by elimination of CO to form, initially, substituted α-carboethoxy carbenium ions although there is significant rearrangement to protonated ethyl esters of olefinic acids prior to further fragmentation.