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High resolution photoabsorption spectra of HC1 and C12 have been measured near the chlorine K edge in the 2810-2850eV photon energy range. Below the C1 K edge, the strongest resonance is interpreted as a simple core excitation into the unoccupied a* valence orbital for both molecules, leading to a markedly repulsive state. Higher resonances due to low lying Rydberg states, are observed in both systems, but with a larger oscillator strength for HC1 as compared to C12. In C12, the o-* orbital is deep enough to avoid any mixing with Rydberg orbitals. In HC1, we observe the dipole forbidden C1 ls--,4s transition which denotes a strong 4s-4p hybridization. Above the C1 K edge, the multiplet features seen for HC1 are analysed in terms of doublecore-valence excited vacancy states. In Ct2, their counterpart are found very close to the ionization threshold because of the deep a* orbital and possibly because the excited core and valence electrons originates either from the same atomic site or from different ones.