Ab initio SCF and CI calculations for the potential surfaces of HCN in ground and various 1(π,π*) excited states are carried out using an AO basis of double-zeta quality augmented with various polarization functions. These results are then combined with transition moment data to allow for a Franck-Condon analysis of the vibrational structure of the lowest three electronic transitions in both HCN and DCN. The resulting intensity distribution is then compared with the corresponding experimental data reported by Herzberg and Innes. This work confirms the earlier conclusion of Schwenzer et al. that the upper state in the [Formula: see text] band system is the 1∑−−1A″species, and not the 1Δ as originally believed. In addition a detailed mechanism for the observed predissociation of the α state is outlined, in which the gradual conversion of the π* MO of bent HCN into a pure hydrogenic 1s AO plays a key role. Arguments are also presented in favor of assigning the [Formula: see text]transition seen in DCN to a 1Δ-21A′ upper state with the same D + CN dissociation limit as for the 1∑−−1A″ species.