A careful kinetic study of the acid hydrolysis of [Co(NH3)(5)Cl](2+) and the selectively deuterated species trans-[Co(NH3)(4)(ND3)Cl](2+), trans-[Co(ND3)(4)(NH3)Cl](2+), and [Co(ND3)(5)Cl](2+) reveals that N-deuteration reduces the hydrolysis rate at both the cis and trans sites. On a per-D basis, the rate reduction is three times more effective for trans-Cl deuteration. This result is contrary to conventional wisdom but has never been tested previously. It appears to be the first example of significant stereoselectivity for the secondary isotope effect in a substitution reaction, in either inorganic or organic systems. An explanation is offered in terms of the effect of D-substitution on zero point energies, the different extents of coupling of NH(D) and Co-Cl vibrational modes according to their stereochemical relationship, and the view that the rate of acid hydrolysis of the [Co(NH3)(5)Cl](2+) involves an aberrant Co-Cl vibration component.