Measured product vibrational distributions of the OH + HCl --> H2O + Cl and OD + HCl --> HOD + Cl reactions show a marked isotopic effect since the HOD product is vibrationally excited while the H2O one is not. Quasiclassical trajectories calculated on a new largest angle generalization of the rotating bond order (LAGROBO) potential energy surface offer a rationale for such an effect in addition to reproducing the measured temperature dependence of the thermal rate coefficient. (C) 2003 Published by Elsevier Science B.V.