The argon-acetaldehyde van der Waals dimer was studied by Fourier transform microwave spectroscopy. Two tunneling motions were observed in the spectrum, an inversion through a planar configuration and methyl internal rotation. A simple deperturbation technique was employed in order to obtain rotational constants for structural purposes. The structure was found to be a nonplanar skew, with the argon binding on top of the (\mathrm{C}-\mathrm{C}-\mathrm{O}) triangle. This was determined by assigning the rotational spectrum of two isotopic species: normal and (\mathrm{Ar} \cdot \mathrm{CH}{3} \mathrm{CDO}). The argon atom is located (3.592(5) \AA) from the acetaldehyde center of mass, and the distances Ar- (\mathrm{O}{\text {cartonyl }}), (\mathrm{Ar}-\mathrm{C}{\text {carbonyl }}), and (\mathrm{Ar}-\mathrm{C}{\text {methyl }}) are (3.59(1), 3.77) (1), and 3.85(1) (\AA), respectively. The dipole moment was determined as (\mu=2.63(2) \mathrm{D}). An induction model was employed to explain the decrease in the dipole moment compared to free acetaldehyde. A dispersion model was used to rationalize the structural data. The binding energy of the dimer was estimated to be (204(1) \mathrm{cm}^{-1}) from centrifugal distortion data and a Lennard-Jones potential. (1994 Academic Press. Inc.