Bitumen is a complex mixture of alkanes, cycloalkanes, aromatics, and substituted fused aromatics that contain a number of heteroatomic functional groups. To improve its properties, bitumen is often modified with an additive that may or may not be reactive. Polyphosphoric acid (PPA) is a reactive additive used to increase the viscosity of bitumen in summer temperatures. Given the complexity of bitumen, the nature of its reaction with PPA is in many respects unclear. To better understand the reactions between bitumen and polyphosphoric acid, we have used Fourier transform infrared spectroscopy and thin-layer chromatography to study the reaction of model bitumen sulfur compounds. We have used saturated, aromatic, and oxidized sulfides, the most common forms of sulfur in bitumen. Only the oxidized sulfide, the sulfoxide group, was found to react upon heating with PPA. This work concludes with a comparison of the relative reactivity of PPA with model functional groups that contain nitrogen, oxygen, and sulfur, and its incidence on the possible reactivity of oxidized and unoxidized bitumens with PPA.