The argon-formic acid van der Waals dimer was studied by Fourier transform microwave spectroscopy. The structure was found to be planar (or very nearly so), with the argon attracted to the acidic hydrogen and carbonyl oxygen. This was determined from assigning the rotational spectrum of three isotopic species : normal, Ar.DCOOH, and Ar.(HCOOH)-C-13. The distance from the argon to the carbonyl oxygen was 3.71(3) Angstrom, to the acidic hydrogen 2.81(3) Angstrom, and to the center of mass of formic acid 3.732(2) Angstrom. The deuterium quadrupole coupling constants were determined for the formic acid-d(1) (DCOOH) complex as chi(aa) = 135(2) kHz, chi(bb) = -75(2) kHz. The dipole moment was also determined as mu = 1.305(1) D. An induction model was employed to explain the decrease in the dipole moment compared to free formic acid. A dispersive model was used to rationalize the structural data. The binding energy of the dimer was estimated to be 144.4(2) cm(-1) from centrifugal distortion data and a Lennard-Jones potential.