The rotational spectra of the Cl-35 and Cl-37 isotopes of the chlorobenzene-argon van der Waals dimer have been assigned using Fourier transform microwave spectroscopy techniques. Rotational constants and chlorine nuclear quadrupole coupling constants were determined which confirm that the complex has C-s symmetry. The argon is over the aromatic ring, shifted from a position above the geometrical ring center towards the substituted carbon atom, and at a distance of about 3.68 Angstrom from it. This distance is 0.1-0.2 Angstrom shorter than the similar distance in the benzene-argon and fluorobenzene-argon complexes. Experimental results are confirmed and explained with the help of second-order Moller-Plesset perturbation calculations using a VDZP+diff basis set. The complex binding energy of the chlorobenzene-argon complex is 1.28 kcal/mol (fluorobenzene-argon, 1.17; benzene-argon, 1.12 kcal/mol) reflecting an increase in stability caused by larger dispersion interactions when replacing one benzene H atom by F or by Cl. The structure and stability of Ar .C6H5-X complexes are explained in terms of a balance between stabilizing dispersion and destabilizing exchange repulsion interactions between the monomers.