A detailed vibrational analysis of the 5515 Å absorption of s-tetrazine (tetra-azabenzene) vapor is attempted. More than 80% of the total intensity of the system can be accounted for by band assignments to a single allowed electronic transition, 1B3 u- 1Ag. The only progression-forming vibration in absorption in ν6 a. Three progressions in ν' 6 a account for all intense cold bands. Isotopic shifts of bands lead to convincing assignments of the two excited-state vibrations upon which ν6 a progressions are based, namely ν6 b and ν8 a. It is shown that in all probability the antisymmetric mode ν6 b is prominent because of vibrational (Fermi) resonance rather than because of vibronic interactions.