Abstract The vapour phase electronic absorption spectrum of 1,2,4,5-tetrafluorobenzene, in the region of the S 1-S 0 band system, has been photographed under conditions of high resolution. Computer simulation of the observed rotational contours has shown that all the most intense bands are type A and, therefore, that the electronic assignment is Ã 1B 2u-X̃ 1A g. The rotational constants in the Ã state are consistent with an out-of-plane angle of the CF bonds of 11 ± 4°. This is consistent with the result obtained by Okuyama et al., from the supersonic jet spectrum, that the fluorine atoms are out-of-plane in a “butterfly” configuration. Observation of hot bands of the butterfly vibration, which are frozen out in the jet spectrum, provides strong support for the W-shaped potential function proposed by Okuyama et al. for this vibration in the Ã state.