By using the time-of-flight technique under crossed-beam conditions, the collision energy dependence of the Ca(D-1) + HCl --> CaCl(A) + H reaction cross-section has been measured over the 0.2-0.5 eV collision energy range and found to have a broad peak at 0.3 eV. An empirical description of entrance covalent channels-including orbital alignment effects due to the open-shell nature of calcium atoms-and a harpooning mechanism, involving ionic charge-transfer states, account for the observed translational energy dependence of the reaction cross section.