We report the absolute absorption cross sections of the fourth. vibrational O-H (5 nu(OH)) overtone in acetic acid using cavity ring-down spectroscopy. For compounds that undergo photodissociation via overtone excitation, such intensity information is required to calculate atmospheric photolysis rates. The fourth vibrational overtone of acetic acid is insufficiently energetic to effect dissociation, but measurement of its cross section provides a model for other overtone transitions that can affect atmospheric photochemistry. Though gas-phase acetic acid exists in equilibrium with its dimer, this work shows that only the monomeric species contributes to the acetic acid overtone spectrum. The absorption of acetic acid monomer.peaks at similar to 615 nm and has a peak cross section of 1.84 x 10(-24) cm(2).molecule(-1). Between 612 and 620 nm, the integrated cross section for the acetic acid monomer is (5.23 +/- 0.73) x 10(-24) cm(2).nm .molecule(-1) or (1.38 +/- 0.19) x 10(-22) cm(2).molecule(-1).cm(-1). This is commensurate with the integrated cross section values for the fourth O-H overtone of other species. Theoretical calculations show that there is sufficient energy for hydrogen to transition between the two oxygen atoms, which results in an overtone-induced conformational change.