The microwave spectra of the argon cyclobutanone van der Waals complex and its three C-13 isotopomers have been assigned in the ground state of the ring puckering vibration. The rotational constants and centrifugal distortion constants for the C-12 species are determined to be A = 3598.5700(4) MHz, B = 1388.4449(1) MHz, C = 1195.5254(2)MHz, Delta(J) = 4.846(2) kHz, Delta(JK) = 36.11(2) kHz, Delta(K) = -38.10(8) kHz, delta(J) = 0.6415-(6) kHz, delta(K) = 20.62(4) kHz, H-k = 0.067(4) kHz. The dipole moments have also been measured: mu(a) = 0.29 D, mu(b) = 2.83 D. The cyclobutanone ring was found to remain planar after complexation with argon. The coordinates of the argon in the monomer＇s principal axis system are a = 0.23, b = 0.55 (across the ring due to large amplitude bending), and c = 3.38 Angstrom (above the ring). The equilibrium position of argon is in the C, plane of the complex, 3.48 Angstrom above the ring and shifted 0.23 Angstrom from the C-2-C-2＇ ring diagonal toward the carbonyl.