Copper phthalocyanine (CuPc) molecules have been deposited under ultra-high vacuum on two kinds of hydrogen-terminated silicon surfaces, namely the H-Si(1 0 0)-2 x 1 and H-Si(1 0 0)-1 x 1 surfaces. The chemical and structural properties of these interfaces were investigated in situ by X-ray photoelectron spectroscopy and X-ray photoelectron diffraction (XPD), and low-energy electron diffraction. Results from X-ray photoelectron diffraction shed light on the growth mechanism of CuPc on these two substrates. At one monolayer coverage the growth mode was characterized by the formation of crystalline 3D nano-islands. The molecular packing deduced from these studies appears very close to the one of the bulk CuPc phase. Interestingly, the growth mode appears significantly different for the two surfaces. Indeed, 3D islands are formed by molecules aligned in a standing manner in the case of H-Si(1 0 0)-2 x 1, and in a flat-lying manner in the case of H-Si(1 0 0)-1 x 1. (C) 2012 Elsevier B. V. All rights reserved.