A novel composition which comprises: A. AN ORGANOTIN SULFUR-CONTAINING COMPOUND, FOR EXAMPLE, ORGANOTIN MERCAPTIDES, ORGANOTIN MERCAPTOACIDS, ORGANOTIN MERCAPTOACID ESTERS, ORGANOTIN SULFIDES, ORGANO THIOSTANNOIC ACIDS AND THE LIKE, B. AN ALKALI OR ALKALINE EARTH METAL SALT OF AN ORGANIC ACID, SUCH AS Group I or Group II-a Periodic Table metal salts of aliphatic monocarboxylic acids, and C. AN ALKALI OR ALKALINE EARTH METAL BASE SUCH AS 1) Group I or Group II-a Periodic Table metal oxides, hydroxides or carbonates and 2) organic over-based complexes of Group I or II-a metal bases. These novel compositions have been found to extend the heat stability of vinyl halide resins in a remarkably effective manner even when used in total proportions and ratios of ingredients varying over a broad range.