The accumulation of phosphogypsum waste from the fertilizer industries, which remain in regulated stacks occupying considerable land resources, is causing significant environment problems worldwide. In that sense, the scientific community is being pressured to find alternative ways for their disposal. In this research, we propose a novel application for phosphogypsum waste, as a modifier of bitumen for flexible road pavements. Viscous flow tests carried out on bitumen modified with a phosphogypsum waste and doped with sulfuric acid demonstrated an extraordinary increase in viscosity, at 60°C, when compared to a counterpart sample which had been modified with gypsum, the main component of phosphogypsum. Similarly, a significant improvement in the viscoelastic response of the resulting material at high temperatures was also found. FTIR (Fourier transform infrared spectroscopy) scans provided evidences of the existence of chemical reactions involving phosphorus, as revealed by a new absorption band from 1060 to 1180cm(-1), related to COP vibrations. This result points at phosphorus contained in the phosphogypsum impurities to be the actual "modifying" substance. Furthermore, no COP band was observed in the absence of sulfuric acid, which seems to be the "promoting" agent of this type of bond.