A thermo-stable, arsenic-free synergistic biocide concentrate composition to afford enhanced antimicrobial properties to a polymer matrix comprising (i) a mixture of trihalomethyl-thio-phthalimide analogue and a second biocide (ii) an antioxidant (iii) a carrier and (iv) optionally, one or more additives. The composition is capable of withstanding high processing temperatures up to about 250° C., substantially without thermal degradation of biocides and discoloration of the polymer matrix employed.
