The self-assembly of monolayers is a highly promising approach in organic electronics, but most systems show weak device performances, probably because of a lack of long-range order of the molecules. The present self-assembled monolayer was formed by a molecule that contains a dimethylchlorosilyl group combined with a quinquethiophene unit through an undecane spacer. This system is the first reported self-assembled monolayer on silicon oxide surfaces that forms two-dimensional crystals. A detailed structural solution is presented based on grazing-incidence X-ray scattering experiments and theoretical packing analysis. By transverse shear microscopy, the shape and size of the crystallites were determined: polygonal shapes with lateral sizes of several micrometers were observed. In situ temperature studies revealed gradual changes of the molecular packing that were irreversible. Melting of the crystal structure was found at 520 K, whereas the self-assembled monolayer remained stable up to 620 K. This work presents unknown structural properties of a self-assembled monolayer revealing insights into layer formation and irreversible evolution upon temperature treatment.