Though surface modifications of organic thin films dramatically improve optoelectronic device performance, chemistry at organic surfaces presents new challenges that are not seen in conventional inorganic surfaces. This work demonstrates that the subsurface of pentacene remains highly accessible, even to large adsorbates, and that three distinct reaction regimes (surface, subsurface, and bulk) are accessed within the narrow thermal range of 30-75 degrees C. Progression of this transition is quantitatively measured via polarization modulation infrared reflection absorption spectroscopy, and atomic force microscopy is used to measure the thin-film morphology. Together, they reveal the close relationship between the extent of the reaction and the morphology changes. Finally, the reaction kinetics of the pentacene thin film is measured with a series of adsorbates that have different reactivity and diffusivity in the thin film. The results suggest that reaction kinetics in the thin film is controlled by both the reactivity and the adsorbate diffusivity in the thin-film lattice, which is very different than the traditional solution kinetics that is dominated by the chemical activation barriers. Combined, these experiments guide efforts toward rationally functionalizing the surfaces of organic semiconductors to enable the next generation of flexible devices