The structure of bitumen/SBS polymer blends and both physical and insitu chemically reacted has been characterized using cryo-scanning electron microscopy. This low-temperature technique has allowed the extension of infrared and fluorescence microscopy studies to a higher resolving power. The microstructure has been imaged at a submicrometer scale. The distribution of the polymer nanoparticles throughout the bitumen matrix has been followed during its fabrication at the high temperature state. The in situ chemically reacted polymer-modified bitumen shows a very good polymer distribution homogeneity compared with the physical blend.