Stabilizer compositions are provided whose ingredients interact synergistically to improve the resistance to deterioration on heating of synthetic resin compositions. The interacting ingredients are (a) a thioether carboxylic acid ester of a polyhydric alcohol having 5 to 15 carbon atoms and 3 to 8 primary hydroxyl groups with a 3-alkylthiopropionic acid having 4 to about 34 carbon atoms, and (b) at least one carbonate ester of an ortho-substituted polyhydric phenol having in the molecule one to three benzenoid rings, two to three phenolic hydroxyl groups, and in each benzenoid ring one to two alkyl, cycloalkyl, or aralkyl groups of which at least one is positioned ortho to a phenolic hydroxyl group.Synthetic resin compositions are provided that are stabilized with the stabilizer compositions disclosed, including olefin polymers, polyamides, acrylic polymers, and vinyl halide polymers.