A polyurethane-forming system having a viscosity at 25° C. of less than 600 mPas for at least 30 minutes, a gel time at ambient temperature of greater than 60 minutes and a water content of less than 0.06% by weight, based on total system weight, is used to produce composites by a vacuum infusion process. This system makes it possible to produce large composites such as wind turbine blades having excellent physical properties.