the efficacy and efficiency of dual flame retardant systems for polyester/cellulosic blend fabrics were studied using an isoperbol calorimeter. blend fabrics containing inherently flame retardant fibers were evaluated. they include blend fabrics with various blend ratios of 900f dacron and pfr rayon, polyester and pfr rayon, and 900f dacron and cotton fibers. several blend fabrics of 900f/cotton were also treated with various levels of phosphoric acid, a condensed phase retardant for cotton. results showed that the dual retardant system can be a viable approach to obtain satisfactory level of flame retardant properties on polyester/cotton blend fabrics. however, the flame retardancy of 900f/pfr rayon blends was not satisfactory, due mainly to insufficient retardant levels in both the 900f and the pfr rayon. data from phosphoric acid treated 900f/cotton blend fabrics indicated that the retardant effect of bromine in the 900f was mainly via a vapor phase mechanism. 13 refs