Monolayers of CF2HCl physisorbed on exfoliated graphite have been studied by x-ray diffraction. The low-coverage 2D solid crystallizes in a herringbone pg structure commensurate (4 x square-root 3) with thc graphite substrate. At higher coverages the lattice is uniaxially compressed. Comparison with the monolayer structures of other methane derivatives and of ethane is made.