The initial stages of the multilayer growth of a model system for molecular solids, namely physisorbed benzene on Ru(001), have been studied in detail by infrared reflection absorption spectroscopy and thermal desorption spectroscopy. A variety of different phases have been discriminated spectroscopically and characterized in situ : the parallel oriented first physisorbed layer which is found to rearrange into a more crowded layer with a high tilt angle at slightly higher coverages; an amorphous layer which grows at low temperatures (T less than or equal to 55 K), and a crystalline layer to which the former converts at elevated temperatures. Clear evidence for structural disorder of the uppermost layer of the crystalline phase is found. The amorphous-crystalline phase transformation is irreversible and the required temperatures vary considerably with the layer thickness. This is attributed to two different processes : at high coverages (Theta greater than or equal to 10 ML) crystallization is possible at low T without mass transport and requires only a reorientation and minor rearrangement of the benzene molecules. Low initial coverages (Theta = 2.5-5 ML) require nucleation and diffusion of benzene molecules to form stable 3D crystallites with the former process acting as the kinetically limiting factor. Particular attention has been devoted to the unravelling of the nature of the metastable state observed in thermal desorption spectroscopy and its transformation into the more stable crystalline phase.