Three different unsaturated polyester resins were modified by toluene diisocyanate (TDI). All the resins studied were commercially available. One, 2, and 3% additions of TDI caused important changes of the resins properties. With an increase of TDI concentration gelation times increased. Suppression of the exotherm peaks-desired effect from practical point of view-was observed. Unfortunately, increase of TDI concentration caused enormous increase of the resins viscosity. Additionally the resins with 3% of TDI exhibit thixotropic properties.