The effect of the counterpoise correction on the geometries and vibrational frequencies of 15 H-bonded systems has been analyzed at the B3LYP and MP2 levels of theory, using the popular 6-31++G(d,p) basis set. The counterpoise correction increases the H-bond distance, decreases the intermolecular stretching frequency and decreases the red-shift of the donor H-X frequency. The observed changes are larger at the MP2 level of theory, for which the basis set superposition error is significantly larger. At both levels of theory, linear relationships between the percentage of BSSE, the relative changes of the H-bohd distances and intermolecular stretching frequencies, offer the possibility of estimating, from one single point counterpoise calculation, the geometry and frequencies that one would obtain from a counterpoise corrected potential energy surface. © 2001 American Chemical Society.