It is well-known that carboxylic acids and phenols are stronger acids than the corresponding aliphatic alcohols. The often accepted explanation is that carboxylate and phenoxy anions are stabilized by resonance. However, from a theoretical point of view, the inductive and resonance effects for these compounds have not yet been well quantified. Making use of generalized multistructural (GMS) wave functions, we computed the gasphase acidities of methanol, ethanol, formic acid, and acetic acid. It is shown that, in the absence of resonance, acetic acid is slightly more acidic than ethanol and that the difference in acidity between formic acid and methanol is only half of the experimental value. Assuming that, in the absence of resonance, the difference in acidities can be attributed mainly to the inductive effect of the carbonyl group, it can be shown that resonance is the dominant effect in determining the greater acidity of the carboxylic acids relative to the corresponding aliphatic alcohols.