Expandable styrene polymer particle compositions containing (1) an expanding agent, which is volatilizable and does not dissolve or may slightly swell the styrene polymer particles, and having a boiling point lower than the softening point of the styrene polymer particles, and (2) a small amount of sulfur containing compound having the following general formula: R3-COOR1 ¦S¦ R4-COOR2 wherein R1 and R2 are the same or different higher alkyl groups having 12 - 18 carbon atoms and R3 and R4 are the same or different lower alkylene groups having 1 - 4 carbon atoms.
