Carbon is incorporated into Si(100) to form a thin polycrystalline layer of SiC by laser melting the Si surface after adsorption of propene in ultrahigh vacuum. The SiC layer of thickness up to 25 nm is polycrystalline. Crystallites of size ≈100 nm are oriented with respect to the Si substrate and exhibit a diffraction pattern in low energy electron diffraction (LEED). The evolution of the surface is monitored in real time by recording the Si transient reflectivity at 675 nm at each laser pulse, and after exposure to the laser by LEED, IR spectroscopy, and atomic force microscopy. The formation of the SiC layer is accompanied by very strong variations of both the static and transient reflectivities, by the growth and narrowing of the IR peak assigned to β SiC, and by the increase of the C incorporation rate. The SiC overlayer is very stable against photodesorption, while initially small amounts of C on Si are photodesorbed in a few laser pulses. Recording the transient reflectivity during processing allows one to evidence that the laser absorption increases drastically as the SiC layer grows, resulting in (undesired) larger melting depth and duration that favor incorporation of C in Si below the SiC layer. SiC layers of improved quality might be obtained by active control of the laser fluence by means of the reflectivity transient.