The origin of the acidity of carboxylic acids and enols has been examined via ab initio MO calculations at the MP2/6-31+G** and Becke3LYP/6-31+G** levels as well as with the CBS-4 and CBS-Q model chemistries. The changes in electron populations that occur on ionization were examined in some detail. In the conversion of vinyl alcohol to its alkoxide ion, the sigma electrons of the proton are donated to the oxygen. The repulsion between the increased sigma density at oxygen and its pi electrons leads to transfer of pi electron density to the C=C double bond. At the same time, the negative charge at oxygen repels the sigma and pi electrons of the attached carbon. A part of the negative charge appears at the hydrogens that are in the nodal plane of the pi system. With longer chains, sigma/pi polarization leads to compensating charge shifts so as to minimize the differences between adjacent carbons. When a carbonyl group is attached, as in carboxylic acids and enols, it serves to stabilize the system by accepting some pi electron density (similar to 0.1 e) and by a stabilizing electrostatic interaction between the positively charged carbonyl carbon and the adjacent negatively charged atom. The stabilization is on the order of 40 kcal/mol for carboxylic acids and 28 kcal/mol for enols. The effect of polar solvents on relative acidities also was examined using a new reaction field model, and experimental observations were reproduced.