The resonance fluorescence spectrum of Cl2 first observed by Rao and Venkateswarlu in 1962, has been photographed at high resolution and analyzed. The assignments of the lines have been revised and new resonance series have been observed. It is shown that the ground state of Cl2, dissociates into Cl atoms with no potential hill and that the dissociation energy isD0 = 19 997.28 cm−1.Vibrational and rotational constants and the R.K.R. curve of the ground state of Cl2 have been determined. The long-range portion of the ground state potential of Cl2 has been examined and shown to be consistent with the predictions of theory and measurements from the B3Π0 potential.