H2O–HCl is studied using a number of basis sets including 6‐31G∗∗ and variants which are augmented by a diffuse sp shell and a second set of d functions on O and Cl. Optimization of the geometry of the complex is carried out including explicitly electron correlation and counterpoise correction of the basis set superposition error (BSSE) at both the SCF and correlated levels. Correlation strengthens and shortens the H bond while BSSE correction leads to an opposite trend; these two effects are of different magnitude and hence cancel one another only partially. ΔH°(298 K) is calculated to be −4.0 kcal/mol, 1/4 of which is due to correlation. Formation of the complex causes the strong intensification and red shift of the H–Cl stretching band normally associated with H bonding, whereas the internal vibrations of H2O are very little affected, except for a doubling of the intensity of the symmetric stretch. With respect to the intermolecular modes, the bends of the proton donor are of higher frequency than those involving the acceptor. While these intermolecular bends are all of moderate intensity, comparable to the intramolecular modes, the H‐bond stretch νσ is very weak indeed, consistent with a principle involving subunit dipoles. All calculated vibrational data are in excellent agreement with the spectra measured in solid inert gas matrices.