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Changes in the m a s spectra with inlet temperature were used in this work to demonstrate the dependence of keto-enol tautomerism of acetylacetone, 3-methyl acetylacetone and 3-allyl acetylacetone on temperature.The largest dependence of temperaturewereshownby the ion [M -42]+' arising froma McLafferty type rearrangement and by the ion [M -Me]+ resulting from simplecc-cleavage. The ion [M -42]+' peak increases with the temperature of the inlet system while the ion [M -Me]+ peak decreases.By assuming that the ion [M -42]+' represents theketo formand thatthe ion [M -Me]+ represents the cir-enol form (stabilized by the hydrogen bond) one sees that the direction of the intensity variation of these peaks with temperature is in accord with the expected change of keto-enol tautomerism with temperature. A quantitative correlation on the basis of the above assumptions is also approached.Recording of the mass spectra of these three p-diketones at different energies of the incident electrons enables us to estimate whether or not the particular ions present in the mass spectra result from energetically favourable processes.The variation of the intensities of the peaks with the temperature of the inlet at different electron energies is also discussed.

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