For the first time the millimeter-wave spectra of the trans-(ClHC)-Cl-35=CHF and trans-(ClHC)-Cl-37=CHF isotopomers have been observed in natural abundance. Many DeltaJ=0, +/-1 DeltaK(-1)=+1 transitions for (ClHC)-Cl-35=CHF and DeltaJ=0 DeltaK(-1)=+1 transitions for (ClHC)-Cl-37=CHF have been detected and assigned. This allowed us to accurately determine the vibrational ground-state rotational constants, quartic and some sextic centrifugal distortion constants, and nuclear quadrupole coupling constants for both Cl-35 and Cl-37. The experimental investigation has been supported by highly accurate theoretical predictions. As far as ab initio computations are concerned, the complete set of cubic and quartic force constants have been evaluated by numerical differentiation of the analytic second-order Moller-Plesset many-body perturbation theory/correlation consistent polarized valence triple zeta second derivatives. The anharmonic part of the force field completes the theoretical study on the equilibrium structure, dipole moment, chlorine quadrupolar tensor, and harmonic force field previously carried out by the same authors. (C) 2004 American Institute of Physics.