The three-dimensional potential energy surfaces of the 1(1)A" and 2(1)A' states and the corresponding transition dipole surfaces from the ground electronic (1(1)A') state of HCN have been obtained from high-level ab initio calculations. Using these surfaces, we investigate the predissociation dynamics of HCN and DCN with a recursive quantum mechanical method. Whereas the perpendicular transition to the 1(1)A" state leads to narrow predissociative resonances, the absorption spectrum of the parallel 2(1)A' <-- 1(1)A' transition is dominated by relatively broad peaks associated with rapid fragmentation. Detailed analysis suggests that the 1(1)A" spectrum of DCN is affected by a 1:2 Fermi resonance between the C-N stretching and bending modes. Both theoretical and experimental evidence suggests that the so-called beta band should be reassigned to the 1(1)A" absorption. In the 2(1)A' state, a strong isotopic effect is found in the fragment internal state distribution and attributed to a Fermi resonance between the H-C and C-N vibrational modes.