Unsaturated polyester resin composition

An unsaturated polyester resin composition comprising A) an unsaturated polyester alkyd prepared by condensation of exclusively or at least 30 mol% of 1,2-butanediol as polyhydric alcohol components, exclusively or in part phthalic anhydride as saturated polybasic acid component and an unsaturated polybasic acid component, and B) an unsaturated monomer which is copolymerisable with component A), where the amount of component B) is from 20 to 55 parts by weight per 100 parts by weight of component A). An unsaturated polyester resin composition of this type has a low viscosity and can easily be filled with glass fibres, pigments, inorganic fillers and the like. In addition, it has the advantage that it smells only a little of the unsaturated monomer and that the effect of evaporation of the unsaturated monomer on the environment and in particular on the processers is reduced.