The structure of copper phthalocyanine (CuPc) and free-base phthalocyanine(H2Pc)overlayers deposited on the (001) and (111)A surfaces of InSb and InAshave been studied by low energy electron diffraction and van der Waals(vdW) intermolecular interaction energy calculations. CuPc forms a(3 × 3) structure on InSb(100) and a ( structure on InAs(100). In contrast,H2Pc forms amixture of a (3 × 3) and ( structure on InSb(100), whilst no ordered structures are formed when grownon InAs(100). These differences are rationalized by vdW intermolecularinteraction energy calculations of the quadratic unit cells of CuPc andH2Pc.For deposition on InSb(111)A, both molecules adopt a structure. This energetically unfavourable hexagonal structure indicates a relatively strongsubstrate–molecule interaction. The lattice dimensions of the Pc unit cells deviate fromthat expected for a hexagonal Pc structure with an intermolecular interaction energyminimum.