Electron-ionization-induced mass spectra of N-monosubstituted 2-phenylacetamides were recorded and their fragmentation patterns were studied by metastable-ion analyses. Representative deuterated analogues of these compounds have also been synthesized and their mass spectra compared with those of the unlabelled parent compounds. The most typical fragmentation for N-alkyl-, N-isoalkyl- and N-cycloalkyl-2-phenylacetamides is cleavage of the bond β to the carbonyl function, resulting in an ion fragment of m/z 92, following the transfer of hydrogen and elimination of a corresponding ketene. The primary fragmentation process for N-aryl substituted 2-phenylacetamides is the loss of an aromatic hydrogen atom from the molecular ion. The other principal fragmentation processes observed with these compounds are discussed.