The pure rotational spectrum of chlorosyl fluoride was studied by microwave Fourier transform spectroscopy and conventional millimeter and submillimeter absorption spectroscopy. More than 100 rotational transitions for each isotopomer were observed involving Jless than or equal to73, 61 and K(a)less than or equal to19, 16 for (FClO)-Cl-35 and (FClO)-Cl-37, respectively. The analysis yielded precise rotational, centrifugal distortion, Cl-35,Cl-37 nuclear quadrupole, and F-19 and Cl-35,Cl-37 nuclear spin-rotation coupling constants. The spin-rotation constants were used to derive nuclear magnetic shielding values. All nonzero elements of the Cl quadrupole tensor were obtained, permitting its diagonalization. The dipole moment was determined by Stark measurements in the millimeter region. The ground-state average structure r(z) and an estimate of the equilibrium structure r(e) were calculated. The properties derived for FClO were compared with those of ClF3 and other related molecules. Structural parameters, harmonic force constants, the dipole moment, and the nuclear quadrupole coupling constants were also evaluated by means of quantum-chemical calculations. The results are in good agreement with experiment.