The recently developed metadynamics method is applied to the intramolecular hydrogen migration reactions of acetone in the gas phase. Comparison of different sets of collective coordinates allows efficient description of the underlying free energy surface. The simulations yielded numerous reactions: the enol-oxo tautomerism, the decomposition of acetone to various products, and rearrangement reactions. On the basis of the calculated activation barriers it is concluded that the enol-oxo tautomerism is the most frequent intramolecular proton-exchange process the acetone undergoes in the gas phase.