The use of helium diffraction patterns to study desorption processes is explored as a novel extension to traditional methods based on helium specular reflection. The sample, cyclooctatetraene adsorbed on Cu(111), provides a rich but complex structure. The modulation of cyclooctatetraene by Cu(111) is manifested as a convolution in the diffraction pattern, displaying an averaged super-cell symmetry of (7 √3 × 7 √3)R30◦. The adlayer expands during isothermal desorption, and the change in lattice constant provides a direct measure of the coverage as a function of time. We find a desorption energy of 0:96 ± 0:01 eV at saturation of the first layer, and an upper limit of 1:62 ± 0:07 eV for isolated molecules. These values, and details of the assigned structure, indicate chemisorbed molecules with a planar conformation.