An infrared absorption spectrum has been observed and analyzed for an Ar-(CO2)-C-12-O-18 van der Waals (vdW) complex in the range of 2312-2314 cm(-1). The effective structure of the complex derived from the obtained rotational constants is in essential agreement with the structure estimated from that of Ar-(CO2)-C-12-O-16 based on the isotope effect on the amplitudes of vdW vibrations. The band origin for the Ar-(CO2)-C-12-O-18 nu (4) mode shows a red shift of 0.4567 cm(-1) with respect to that for (CO2)-C-12-O-18 nu (3) mode; the change in the intramolecular potential of CO2 upon complex formation with Ar is determined from the isotope effect on the band-origin shift.