Abstract: A blowing agent concentrate comprising a chemical blowing agent and a thermoplastic composition vehicle therefore, said thermoplastic composition being a saturated polyester and/or copolyester having a crystalline melting point between 100.degree. C. and 220.degree. C. and a reduced viscosity of 0.4 dl/g to 1.6 dl/g; the use of such blowing agent concentrate in the formation of foamed articles of high molecular weight polyalkyleneterephthalate which articles can be filled and/or reinforced and/or flame proofed and can contain other additives. Particularly contemplated is the use of such blowing agent concentrate in the formation of glass fiber reinforced structural foam moldings of polyalkyleneterephthalates especially polytetramethylene terephthalates.