A molten material is prepared by heating a mixture of (A) a polyhydric alcohol containing a neopentyl structure such as pentaerythritol, dipentaerythritol or tripentaerythritol which is less compatible with a halogen-containing resin and (B) a partial ester of said polyhydric alcohol and a carboxyl group-containing compound which has good compatibility with the halogen-containing resin to the molten state. The molten material functions as a stabilizer assistant which markedly increases the thermal stabilizing effect of divalent metal stabilizers for the halogen-containing resin.
