Foam building materials are used in roofing and water proofing membranes, insulation, pipe covers, vapour barriers, crack suppressant membranes, and as installed flooring protection. In this study, new stable bituminous foams have been developed, in a two-step method, using a reactive polymer, synthesized from the reaction of 4,4'-diphenylmethane diisocyanate with a low molecular weight polypropylene glycol (MDI-PPG). Firstly, a modified bitumen was obtained by adding 10 wt.% MDI-PPG to bitumen, at 90 degrees C. In a second step, an excess of water was added, which reacted with the remaining -NCO groups and generating CO(2), which was used as a chemical foaming agent to produce stable bituminous foams. In general, polymer addition improves the bituminous foam rheological properties, mainly in the high in-service temperature region. In this range of temperature, the rheological properties are clearly affected by reaction time. (C) 2010 Elsevier Ltd. All rights reserved.