Cavity ringdown laser absorption Spectroscopy has been used to record spectra of ((HCl)-Cl-35)(2) and its Cl-substituted mixed dimers at the first overtone of the free hydrogen stretch (2 yl). The dimers were produced in pulsed planar supersonic expansions. Significant quenching of interconversion tunneling (i.e., which exchanges the roles of H-bond donor and acceptor) has been observed. Thus, the (HCl)-Cl-35-(HCl)-Cl-37 and (HCl)-Cl-37-(HCl)-Cl-35 heterodimers are distinguished in the 2v(1) eigenstates, which is not the case for the ground and HCl-stretch fundamental eigenstates because of facile tunneling mixing.