The dissociative photoionization of CH2Cl2 in a region similar to11-20 eV was investigated with photoionization mass spectrometry using a synchrotron radiation source. Major fragment ions CH2Cl+, CHCl+, and CCl+ were observed with their respective appearance energies at 12.08, 12.46, and 15.96 eV. Relative branching ratios as a function of photon energy were determined. Dissociative photoionization channels for formation of these three fragment ions are proposed based on comparison of determined appearance energies and energies predicted with the Gaussian-2 calculations. The principal dissociative process is direct breakage of the C-Cl bond to form CH2Cl++Cl; at greater energies formation of CHCl+ takes place via the CHCl++HCl channel. Formation of CCl+ near the threshold at 15.96 eV may proceed via the CCl++HCl+H channel rather than CCl++H-2+Cl channel, but at greater energies the latter dominates. Enthalpy of formation at 0 K of CHCl+ is revised to 288.0+/-0.7 kcal mol(-1) based on our experimental results; the adiabatic ionization energy of CHCl is accordingly revised to 9.12+/-0.12 eV. (C) 2003 American Institute of Physics.