Polyphosphoric acid (PPA) is used to modify bitumen, but the chemical reactions responsible for the change in bitumen properties remain, for the most part, undetermined. In an effort to better understand these reactions and phosphorylation in particular, four bitumen model compounds with various oxygenated functional groups were heated with PPA in conditions typical of those used to prepare PPA-modified bitumens, namely, bisphenol A (phenol), butyl phenyl ether (ether), acetophenone (ketone), and benzoic acid (carboxylic acid). The reactions were followed by thin-layer chromatography and infrared spectroscopy. It was found that the extent of the reactions decreased as bisphenol A > acetophenone > benzoic acid >> butyl phenyl ether. The detailed analysis of the infrared spectra revealed that the hydroxyl group of bisphenol A was not phosphorylated by PPA. Instead, it facilitated the scission of the bisphenol into fragments of lower molecular weight. The reaction of acetophenone had the reverse effect, because it led to condensation. Benzoic acid and butyl phenyl ether were immune to the effect of PPA when heated alone with PPA, but when they were heated together, they condensed into higher molecular-weight aryl-aryl ketones and esters. These findings are consistent with changes known to occur in PPA-modified bitumen. For instance, the breaking down of bisphenol may relate to the decrease in the molecular weight of asphaltenes, whereas the intermolecular reactions translate into an extension of the bitumen molecular backbone that helps explain bitumen stiffening. The key finding, however, is the lack of hydroxyl phosphorylation in phenol, bisphenol, and benzoic acid, which indicate that phosphate esters cannot be held responsible for the modification of bitumen with PPA.