Direct adsorption of phenylacetylene on clean silicon surface Si(100)-2 x 1 is studied in ultrahigh vacuum (UHV). The combination of scanning tunnel microscopy (STM)and surface differential reflectance spectroscopy (SDRS) with Monte Carlo calculations are put together to draw a realistic kinetic model of the evolution of the surface coverage as a function of the molecular exposure. STM images of weakly covered surfaces provide evidence of two very distinct adsorption geometries for phenylacetylene, with slightly different initial sticking probabilities. One configuration is detected with STM as a bright spot that occupies two dangling bonds of a single dimer, whereas the other configuration occupies three dangling bonds of adjacent dimers. These data are used to implement a Monte Carlo model which further serves to design an accurate kinetic model. The resulting evolution toward saturation is compared to the optical data from surface differential reflectance spectroscopy (SDRS). SDRS is an in situ technique that monitors the exact proportion of affected adsorption sites and therefore gives access to the surface coverage which is evaluated at 0.65. We investigate the effect of surface temperature on this adsorption mechanism and show that it has no major effect either on kinetics or on structure, unless it passes the threshold of dissociation measured at ca. 200 degrees C. This offers a comprehensive image of the whole adsorption process of phenylacetylene from initial up to complete saturation.