The growth of ultrathin chloro(subphthalocyaninato)boron (SubPc) films on Au(111) single crystals was investigated with low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). The samples were prepared under ultrahigh vacuum conditions by means of organic molecular beam epitaxy technique. Due to the high mobility of the SubPc molecules on the Au(111) surface, a highly ordered growth of SubPc was only observed for a coverage ≥ 1 ML. The completed first monolayer consists of large domains which cover entire gold terraces. A detailed analysis of the STM and LEED data revealed that the first molecular layer is composed of two orientations of a rectangular 12 × 24 Å lattice with two molecules per unit cell. The interpretation of the LEED data required a calculation of spot intensities based on the kinematic approximation. While the molecules in the first monolayer are standing upright with their isoindolyl groups being attached to the surface, the molecules in the second layer grow upside-down on top of the first layer, forming a commensurate (2 × 2)R45° lattice.