The microwave (4-20 GHz range) and infrared (HCl and DCl stretch ranges) spectra of six isotopic species of the CH3Cl-HCl hydrogen bond complex have been recorded for the first time and analyzed with the support of high level ab initio calculations (MP2 and CCSD(T) levels). Accurate molecular parameters, including rotational, quartic centrifugal distortion, and nuclear-quadrupole coupling constants, vibrational frequencies, and anharmonic coupling constants, are presented in this paper. These parameters have then been used to estimate the hydrogen bond geometry and confirm the strong coupling between intramolecular and low frequency intermolecular modes. Experimental and theoretical evidence, in agreement with each other, tend to point out a free rotation of the CH3Cl unit in the complex, emphasizing the very peculiar dynamical properties of a hydrogen bond and, consequently, the necessity of taking those effects into account to correctly model the intra- and intermolecular interactions.