Complete assignment of the fundamental vibrations of the two benzene valence isomers, Dewar benzene (bicyclo[2,2,0]hexa-2,5-diene) and benzvalene (tricyclo[3,1,0,02,6]hex-3-ene) based on infrared and Raman spectra are reported. Both molecules exhibit abnormal C=C stretching frequencies indicative of σ-π interaction between the strained bridge bond(s) and the π-bonds. The ‘wing flapping’ mode of Dewar benzene occurs at 381 cm-1. © 1975, CSIRO. All rights reserved.