We report for the first time the successful application of rotationally resolving spectroscopy to the acetic acid dimer. We report a study by femtosecond degenerate four-wave mixing (fs DFWM) in a gas cell at room temperature under supersonic jet conditions (ca. 90 K). From the fitting analysis, two centrifugal distortion constants and the sum of the rotational constants (B + C) = 1657.2 ± 1.3 MHz and (B + C) = 1445.2 ± 1.1 MHz were obtained for (CH3COOH)2 and (CD3COOD)2, respectively. These (B + C) values are in good agreement with the results of ab initio geometry optimizations. A detailed analysis of the experimental spectrum produced constraints for the polarizablity tensor and the molecular asymmetry (B − C). The fs DFWM spectroscopic behaviour of acetic acid is compared with previous results obtained for formic acid.