Optimized geometries and harmonic vibrational frequencies have been obtained for neutral and protonated acetic acid using quadratic configuration interaction methods. The proton affinity of acetic acid calculated at the QCISD(T)/6-311++G(3df,3pd)//QCISD/6-311+G(2d,2p) level of theory is 187.9 +/- 0.7 kcal mol(-1), in excellent agreement with the experimental value of 187.3 kcal mol(-1). Thermochernical calculations using the CBS-Q, CBS-QB3, CBS-A-PNO, G2 and G2(MP2) model chemistries also display excellent agreement with the experimental proton affinity, suggesting that these model chemistries will provide accurate proton affinities for carboxylic acids that are too large to treat with high level ab initio methods. (C) 2002 Elsevier Science B.V. All rights reserved.