Asphalt, roofing, and sealing markets strongly demand new materials for being used, for instance, as bituminous coating membranes. In that sense, this paper deals with the effect on these bitumen-based composites of the addition of a MDI-PEG reactive polymer, synthesized by the reaction of 4,4'-diphenylmethane diisocyanate with a low molecular weight poly(ethylene glycol). A short-term modification, taking place during mixing, and a long-term bitumen modification, which develops for a long period of curing (up to several months), have been identified. Both of them result from chemical reactions between NCO groups in the MDI-PEG and some of the bitumen compounds, mainly, the resin fraction. However, long-term bitumen modification seems to be related to the polymer-bitumen reactions but influenced by the environmental conditions (probably due to air moisture). This fact may be used to improve the manufacture of new materials with suitable properties according to their final application.