The origin band of the 2700 Å system of chlorobenzene has been rotationally analysed by the asymmetric rotor band contour method. It is shown to be a typeBband of a prolate asymmetric top which requires the electronic assignment of the system to be1B2-1A1. The excited state rotational constants are:A′=0·1797 ± 0·0001 cm-1,B′=0·05230 ± 0.00002 cm-1,C′=0.040512 ± 0.000007 cm-1. These can be interpreted in terms of a contraction of the C-Cl bond of about 0·04 Å and an expansion of the benzene ring similar to that in the1B2uexcited state of benzene. The origin of the 2700 Å band is shown to be at 37 048·2 ± 0·2 cm−1. Strong typeAbands involving theb2vibrationv29are also assigned from their rotational contours. The 291level is shown to be in Fermi resonance with the 141201level. Of the eight prominent sequence intervals associated with the origin band, five are interpreted with considerable certainty.