In this extension to a classic physical chemistry experiment, students record the proton nuclear magnetic resonance spectra of the [beta]-diketones 2,4-pentanedione, 3-methyl-2,4-pentanedione, and 3-chloro-2,4-pentanedione to investigate the effect of substituents on keto-enol tautomerization equilibria. From the integrated intensities of keto and enol methyl proton peaks, students calculate the equilibrium constant for each 2,4-pentanedione. The students then use the effects of electron-donating and electron-withdrawing substituents to elucidate the nature of keto-enol equilibria. (Contains 2 tables.)