Abstract.We have calculated, using second order perturbation theory, the two photon ionization cross section of a K-shell electron of chlorine for E = 1.6 keV incident photons. Two classes of intermediate states must be considered, those in which a ls electron moves to an empty p-orbital, and those in which an electron from an occupied p-orbital moves into the continuum. The first class of intermediate states is followed by the ejection of a pelectron into the continuum. The second class of intermediate states is followed by the transfer of a ls electron into an empty p-state. The largest contribution comes from 3p --, d-continuum followed by ls ~ 3p transition.Our result is ~(2)/I = 2.06 × 1041 cm4/W where o -(2) is the two photon ionization cross-section and I is the light intensity.