Mass spectrometric evidence of tautomerism is reported for b-ketoamides. The most abundant tautomers are ketoamido and Z-enolamido (both forming internal hydrogen bonds). The influence of temperature on the enol/keto abundance ratios is studied in order to estimate the enthalpy difference for tautomeric equilibria. Theoretical calculations have been done in order to support experimental determinations. There are consistent trends between theoretical and experimental data: Enolization is favored by electron-withdrawing substiuents, bulky groups in a position and temperature increase.