The (C2H4O2) potential energy surface related to the thermal decomposition of acetic acid has been reinvestigated using ah initio MO calculations. Thermochemical parameters have been estimated using approximate QCISD(TC)/6-311++G(d,p) + ZPE calculations based on MP2/6-31G(d,p) geometries. In contrast to a previous theoretical study, our results show that the decarboxylation of acetic acid (CH3COOH --> CO2 + CH4) requires an activation energy similar to that of its dehydration (CH3COOH --> H2CCO + H2O). It is also confirmed that the direct dehydration is less favored than the two-step process involving the 1,1-ethenediol intermediate. Higher level calculations predict that the latter is 115 +/- 10 kJ mol(-1) higher in energy than acetic acid with a heat of formation at O K,Delta H-f,0(o) (H2C=C(OH)(2))=-303+/-10 kJ mol(-1). The classical barrier heights (Delta H-# at 0 K in kJ mol(-1)) have been estimated as follows : 301 for decarboxylation, 317 for direct dehydration, 311 for 1,3-H shift, and 312 for dehydration of ethenediol. The ratio of the rate constants k(dehydration)/k(decarboxylation), calculated using a quantum RRK approach, is about 2 to 9 in the temperature range 1300-1800 K, in agreement with experiment. Thus, there is no need to invoke a bimolecular water-catalyzed decarboxylation mechanism as proposed in an earlier theoretical study.