Rotational constants for CN(X)-H-2/D-2(j(2) = 1), derived from electronic spectra, exhibit an unusually small dependence on H-2/D-2 isotopic substitution. In the present study, variational bound state calculations were carried out to examine the origin of this effect. A four-dimensional potential energy surface [J. Chem. Phys. 110 (1999) 10380] was used for this model. The results show that the lowest energy rotational manifolds of CN(X)-H-2/D-2(j(2) = 1) are significantly influenced by Coriolis state mixing. This perturbation is most pronounced for CN-H,, and is identified as the cause of the anomalous isotope effect.