Keto-enol tautomerism refers to chemical equilibrium between a keto and an enol forms. It is usually assumed that the interconversion of the two forms involves a simple intramolecular proton transfer and the subsequent reorganization of the binding electrons leading to a double bond displacement (Eltekov-Erlenmeyer rule) [1]. In the present work, the possible intramolecular mechanism of tautomeric reaction between the unsaturated 2-methylprop-1-en-1-ol alcohol (I) and 2-methylpropanaldehyde (II) was studied according to Scheme below, by ab initio SCF-RHF calculations using the quantum-chemical program GAUSSIAN 09 [2] and the 6-31G basis set.Fig. 1. Energy profile of reactionfrom 2-methylprop-1-en-1-ol to2-methylpropan aldehyde. The calculated activation energy of this reaction is 375,95 kJ/mol. The isomerization process is exothermic with energy release of 30,72 kJ/mol. Thus, one can conclude that the keto isomer (2-methylpropan aldehyde)is more stable than corresponding enol form. However, the calculated energy barrier between the isomers is too high for the free interconversion between them, and in this case the process can be started only with the help of a catalyst.The transition state for the intramolecularproton transfer (TS in Fig. 1) was localized, and the intrinsic reaction coordinate (IRC) from (I) to (II) was calculated.Calculations have shown that TS is indeed a transition state with one imaginary frequency 2667,25i cm-1.