Industrial-grade bitumen (85:25) was treated with 4,4'-diphenyl methane diisocyanate as a blocking agent to make it compatible with polyurethane resin. Optimization of treatment conditions for the bitumen such as isocyanate dose (similar to 3 wt % of bitumen), reaction temperature (180 degrees C), and treatment time (120 min) was done on the basis of estimating its residual acid value and unreacted -NCO groups. Formation of the urethanized bituminous species after treatment resulted in a reduction in the glass-transition temperature of the base bitumen from -9.63 degrees C to -17.09 degrees C and in moisture vapor transmission from 16.95 to 12.21 g 24 h(-1) m(-2). The bituminous networks were prepared from these treated/SBS-modified treated bitumen and polyurethane prepolymers by in situ and conventional liquid blending methods. Lack of low-temperature flexibility in the bituminous network made from the blending method restricted its use for waterproofing/sealing purposes. Modulated differential scanning calorimetry showed the presence of two overlapping glass-transition temperatures and an endothermic peak in the in situ prepared networks similar to the base bitumen, evidence of a close intermixing of the bitumen constituents with the polyurethane phases. Rheological studies revealed that the SBS-modified bituminous polyurethane network exhibited superior behavior than that of other systems in terms of stiffness and elasticity over a wide range of frequencies. The compounded bituminous networks satisfied the requirements of standard specifications and can be suitably used for waterproofing purpose and sealing of concrete joints. (c) 2006 Wiley Periodicals, Inc.