The vapor absorption spectra of 1A″(nπ ∗) ← S 0 for benzaldehyde- h 6, benzaldehyde-1- d, benzaldehyde-4- d, benzaldehyde-3,5- d 2 and benzaldehyde- d 6 have been obtained, and vibrational assignments made in the 3700–3400 Å region. In all the spectra, the aldehyde torsional mode and the aldehyde out-of-plane mode show weak vibronic activity. In the 600–900 cm −1 region where ring hydrogen modes dominate, the strong band at 730 cm −1 in benzaldehyde- h 6 and at 736 cm −1 in benzaldehyde-1- d is assigned as the ν 11 benzene analog, and the weaker band at ∼860 cm −1 in both compounds to the ν 17b analog. Ring deuteration changes the form of the normal coordinates so that the strong band at 708 cm −1 in benzaldehyde-4- d, 668 cm −1 in benzaldehyde-3,5- d 2 more closely resemble the ν 4 benzene analog. A second moderately strong band appears at 812 cm −1 in benzaldehyde-4- d, 854 cm −1 in benzaldehyde-3,5- d 2- and in benzaldehyde- d 6 at 724 cm −1, and is assigned to the ν 17b benzene analog. The out-of, plane aldehyde hydrogen wag is not resolved in the vapor spectra indicating at the most, very weak activity. Two important conclusions arise from the vibronic assignments. Since the ring out-of-plane hydrogen modes are most effective in the vibronic intensification of the 1 A″(nπ ∗) ← S 0 transition of benzaldehyde, there is considerable n-electron delocalization into the ring appropriate for describing the nπ ∗ state within the virtual orbital model. The highly active normal modes ( ν 4 in benzaldehyde-4- d, and -3,5- d 2, ν 11 in benzaldehyde- h 6) involve substantial out-of-plane displacements of both the carbon and hydrogen atoms para to the aldehyde group. In contrast; in the very weakly active mode ν 10a in benzaldehyde- h 6 and -1- d these displacements either vanish or are very small supporting an n-electron density extending through the ring to the para position.