Compositions and methods are described wherein a styrenic polymer is rendered flame retardant by incorporation therein before, during or after its formation, a combination of (a) at least one organic phosphorus additive that (i) is halogen-free, and (ii) is composed solely of carbon, hydrogen, and phosphorus, and optionally and preferably one or more of the elements nitrogen, oxygen, and sulfur; and (b) elemental sulfur; in proportions such that (i) the sulfur content from b) is less than 2 percent by weight of the weight of the styrenic polymer plus the weight of a) and b), and (ii) the styrenic polymer plus ingredients a) and b) can provide test specimens which exhibit an LOI of at least 24 if tested in accordance with ASTM Standard Test Method D 2863-87.