An ab initio configuration interaction (CI) study including spin-orbit coupling is carried out for numerous valence and ion-pair states of the ClF molecule by employing relativistic effective core potentials. The computed spectroscopic constants are in good agreement with available experimental data for both valence and ion-pair states. The present calculations accurately reproduce the measured spin-orbit splittings for the first (3)Pi multiplet and confirm the recent corrected spectroscopic parameters for the A (3)Pi(1) state [V. A. Alekseev, D. W. Setser, and J. Tellinghuisen, J. Mol. Spectrosc. 195, 162 (1999)]. The electronic structure of the six lowest ion-pair states of ClF which correlate to the Cl+(P-3)+F-(S-1) limit is analyzed in detail. It is shown that strong homogeneous perturbations of the E0(+) and f0(+), beta 1, and G1 states are mainly caused by an avoided crossing of the 3 (3)Pi and 2 (3)Sigma(-) parent Lambda-S states. The electric-dipole transition moments have also been calculated for transitions from the E and f ion-pair states to the lower-lying 0(+) valence states. Emission spectra for the bound-bound and bound-free transitions have been computed on this basis and found to be in good agreement with the measured spectra. A similar analysis for the ion-pair D'2, beta 1, and G1 states will be presented in the subsequent study.