Abstract Potential curves for the ground and all excited singlet states which dissociate into the ground states of the atoms, Cl( 2p u) and F( 2P u), or into the ionic species, Cl +( 1D g) and F −( 1S g), have been calculated by means of the multi-reference configu interaction (MRD CI) method. The Gaussian basis set consists of a total of 70 contracted functions, including Rydberg and bond functions. The calculations show that adequate treatment of Rydberg-valence mixing is essential for an understanding of the ClF electronic absorption spectrum. The calculated D o value for the ground state is 2.47 eV compared to the experimental value of 2.617 eV. Transitions of potential spectroscopic interest include the dipole allowed 1Σ + → 1/gP transition, having a vertical transitio energy of 8.92 eV, and the allowed 1Σ + → 1Σ + transition, with a vertical transition energy of 10.3 eV.