The influence of the molecular weight of poly(2,6-dimethylphenylene oxide) (PPO) on the flame retardancy of ethylene-propylene-diene-modified elastomer (EPDM) formulations containing melamine, kaolin, and PPO formulations was studied. The influence of the molecular structures of various char-forming polymers on their flame-retardant effect was also investigated. PPO resins having number-average molecular weight (M-n) from 3200 to 24,800 and weight-average molecular weight (M-w) 9000 to 58,400 affected the oxygen index (OI) values and UL 94 ratings of EPDM formulations, and the preferable molecular weight was found to be about M-n 13,300 and M-w 29,200. Among the char-forming polymeric additives studied, PPO was most effective in providing flame retardancy. The concept of char-forming rate is proposed to explain the variation in the observed flame retardancy. Higher char-forming rate (in contrast to char yield) correlated well with higher OI and better UL 94 ratings in these systems. The melting-before-charring character of char-forming polymers was another important factor that appeared to control char morphology and thus flame retardancy.