The two-photon spectrum of the acetaldehyde-Ar complex has been measured in the vicinity of the (n,3s) Rydberg transition of the acetaldehyde monomer. The cluster spectrum is blue shifted by 14 cm(-1) from the monomer origin. The vibrational analysis yields harmonic frequencies of 32, 27, and 21 cm(-1) for the intermolecular modes. The two high-frequency modes are Franck-Condon active, indicating a substantial displacement of the minimum configuration with respect to the ground-state equilibrium structure. The abrupt disappearance of the vibrational structure at energies of about 190 cm(-1) above the origin is interpreted as the dissociation limit of the complex.