It is well known that mass spectra of lactams can provide valuable information with regard to lactam- lactime equilibria occurring in gas phase. Mass spectra of selected lactams and thiolactams have been analyzed and specific fragmentation assignments are analysed in this article in order to characterize and weigh co-existing lactam and lactime tautomers. The predictive value of this methodology is not only supported by the influence of these compounds nature and size of the substitution on these equilibria but also by the good correlation found between the selected fragments abundances ratio and Molecular Orbital semi-empirical calculations via the standard Austin Method 1 (AM1) of the corresponding heats of tautomerization. The results indicate that the lactam-lactime and thiolactam-thiolactime equilibrium can be studied by mass spectrometry and ionization in the ion source has negligible effect on the position of that equilibrium.