2-Hydroxyoxol-2-ene (C-5-1), the enol tautomer of gamma-butyrolactone, was generated in the gas phase as the first representative of the hitherto elusive class of lactone enols and shown by neutralization-reionization mass spectrometry to be remarkably stable as an isolated species. Ab initio calculations by QCISD(T)/6-311+G(3df,2p) provided the enthalpies of formation, proton affinities, and gas-phase basicities for gaseous lactone enols with four- (C-4-1), five-(C-5-1), and six-membered rings (C-6-1). The acid-base properties Of C-4-C-6 lactones and enols and reference carboxylic acid enols CH2=C(OH)(2) (3) and CH2= C(OH)OCH3 (4) were also calculated in aqueous solution. The C4-C6 lactone enols show gas-phase proton affinities in the range of 933-944 kJ mol(-1) and acidities in the range of 1401 -1458 kJ mol-1. In aqueous solution, the lactone enols are 15-20 orders of magnitude more acidic than the corresponding lactones, with enol pK(a) values increasing from 5.6 (C-4-1) to 14.5 (C-6-1). Lactone enols are moderately weak bases in water with pK(BH) in the range of 3.9-8.1, whereas the lactones are extremely weak bases of pK(BH) in the range of - 10.5 to -17.4. The acid-base properties of lactone enols point to their high reactivity in protic solvents and explain why no lactone enols have been detected thus far in solution studies.