6,13 Pentacenequinone (PQ) ultrathin films (3 and 5 nm nominal thickness) have been grown by means of ultrahigh vacuum deposition onto 100 nm thick SiO2/Si(100). The structure and morphology of the thin films have been studied with field emission-scanning electron microscopy, tapping mode atomic force microscopy, and x-ray diffraction. The growth begins with PQ molecules standing almost upright (with respect to the substrate) and aggregating into two-dimensional 3.62 nm thick pseudodendritic islands. The islands are characterized by two preferential growth directions (at 117°), a clear evidence of a chiral growth. The thickness of the islands and the angle formed by the two directions of preferential growth, allow a straightforward assignment to a PQ initial growth in the “bulk” phase. Above the critical thickness of 3.62 nm the PQ growth proceeds in a Stranski−Krastanov mode, with the formation of “bulk” and “thin-film” phase crystallites.