The electron ionization mass spectra of the 1-phenyl-, 1-benzyl- and 6-benzyl-1-phenyl-2,3-dihydroimidazo[1,2-a]pyrimidine-5,7(1H,6H)-dione derivatives were recorded at 70 eV to find out the effects of substituents on their fragmentations. Fragmentation pathways were studied using B/E and B(2)/E scans. Some fragmentations involved the loss of C(3)HO(2) or carbon suboxide. The possibility of keto-enol tautomerism was also studied. For comparison selected compounds were studied using (1)H and (13)C NMR spectroscopy to reveal the presence of possible tautomerism. Some ions including [M-OH](+) and [M-HCO](+) and NMR results indicate that the enol form is predominant both in the gas and in the liquid phase.