The unstable product found in the chlorination of formic acid has been identified as formyl chloride by microwave spectroscopy. The lifetime of this molecule was about 3 min in the waveguide cell at dry ice temperature. The rotational constants of seven isotopic species have been measured and analyzed, yielding moments of inertia from which a molecular structure was calculated. The bond lengths found are (in Å)C–Cl=1.760, C–O=1.188, and C–H= 1.096. The bond angles are HCCl=109°58′ and HCO=126°29′. Planarity was established by the analysis of the inertia defects. The nuclear quadrupole coupling constants in the C–Cl bond-axis system are χzz= −59.8 MHz and η=−0.30. From the analysis of the Stark effect under a weak electric field condition, the dipole moment has been determined as ‖μa‖=0.3±0.2 D, ‖μb‖=1.6±0.2 D, and ‖μtotal‖ = 1.6±0.2 D.