A masterbatch composition suitable for use as a flame retardant in extruded polymer foams, and process for manufacturing the same, and extruded foams containing same; the composition comprising: (a) 20 to 40 parts by weight base resin comprising styrene homopolymer or copolymer; (b) 1 to 16 parts by weight acid scavenger comprising an epoxy-based compound; (c) 2 to 6 parts by weight antioxidant comprising an alkyl or aryl phosphite; and (d) 45 to 60 parts by weight flame retardant comprising a non-hexabromocyclododecane(HBCD) brominated polymer or copolymer, wherein the amounts of (a), (b), (c), and (d) total 100 parts by weight; and (e) 0.6 to 10 parts by weight of pH moderator, based on 100 parts of (a) base resin plus (e) water soluble pH moderator.