This work deals with the influence that bitumen colloidal nature exerts on the rheological properties of bitumen samples modified by isocyanate-based reactive polymers. Shear rheology tests, modulated differential scanning calorimetry (MDSC), chemical characterization by TLC-FID, and AFM microstructural analysis were carried out on four different 150/200 penetration neat bitumen samples and the corresponding MDI-PPG (a low molecular weight polypropylene glycol functionalized with a polymeric 4,4'-diphenylmethane diisocyanate) modified binders. The results obtained demonstrate that the bitumen modification degree depends on bitumen reactivity and microstructure. Thus, the highest modification capability is obtained with neat bitumen Samples that exhibit both a well-developed three-dimensional network and a high chemical reactivity with the isocyanate groups. The results obtained may be used to improve the performance of these materials, according to their final application.