Rotational spectra of cyclopentene oxide and the argon-cyclopentene oxide van der Waals complex were Studied using pulsed-jet Fabry-Purot Fourier transform microwave (FTMW) spectroscopy. Spectra of the parent along with those of the C-13 and O-18 singly Substituted isotopologues, in natural abundance, of the monomer and of the complex were measured in the frequency region of 5-26.5 GHz. The complete heavy atom Substitution structure was determined for the monomer and complex. The boat structure for cyclopentene oxide was confirmed with naturally abundant C-13 and O-18 isotopes. For the argon cyclopentene oxide complex, both a and b-type transitions were observed and the rotational constants for the all-C-12 O-16 isotopologue were determined to be A = 3268.254(2), B = 993.345(1), and C = 950.430(9) MHz. The r(0) coordinates of the argon in the principal axis system of cyclopentene oxide are a = 0.27, b = 0.42, and c = 3.91 angstrom, such that the argon is exo to the boat of the ring and oil the opposite side of the ring front the oxygen and is 0.42 angstrom off to the side and 0.27 angstrom from the center of mass toward the back end of the ring (again away from the oxygen). Large amplitude van der Waals bending vibrations require an averaging model to account for differences between the observed complex and monomer planar moments of inertia.