Some new examples of favorable interactions between additives in flame retardant formulations have been found. These include the following: phosphorus combinations such as triphenyl phosphate with a tetraphenyl phenylene diphosphate, and char-former combinations such as novolacs with polyphenylene oxide in styrenics, melamine with polyphenylene oxide and kaolinite in polyolefin elastomers; and a neopentyl phosphonate ester with a melamine phosphate in ethylene-vinyl acetate. Useful interactions were detected by various tests and data analysis methods including multiple regression, and further elucidated by various analytical methods. Some indications of possible modes of action and explanations for the interactions are presented. Barrier formation appears to play an important role in these three cases, although vapor phase action (possibly physical) is also noted. Rate of char formation is suggested as an important variable. The systematic use of combinations of differently functioning flame retardants constitutes the ‘systems approach’ which is advocated for the solution of retardancy problems.