Molecular dynamics of cyclohexenone derivatives of (2E,4E)-1-(2-hydroxy-5-methylphenyl)-5-phenyl-2,4-pentadiene-1-one (chalcone) has been investigated in solutions using NMR. The results confirm the formation of O-H⋯O intramolecular hydrogen bond and the presence of keto–enol tautomeric transitions in the (E)-6-acetyl-3-(2-hydroxy-5-methylphenyl)-5-styryl cyclohex-2-en-1-one. The free energy of activation for the keto–enol tautomeric transitions has been calculated. The keto–enol tautomerism in the investigated compound has been also confirmed by chemical method. Copyright © 2013 John Wiley & Sons, Ltd.