The effects of the species and content of a swelling agent on the molecular weight of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) synthesized in an aqueous medium were studied. It was found that the molecular weight of PPO increases after introducing a certain amount of the swelling agent during the oxidative polymerization of 2,6-dimethylphenol (DMP). T-g of the PPO/ swelling agent mixture decreases with the increase of the swelling agent content, and the relation between T-g of the PPO/swelling agent mixture and the swelling agent content obeys Fox equation. After the introduction of the swelling agent during the oxidative polymerization of DMP, the molecular weight of PPO is correlated with T-g of the PPO/swelling agent mixture and it was revealed that T-g plays an important role in the molecular weight of PPO synthesized in the aqueous medium. The same molecular weight of PPO can be obtained only if T-g of the PPO/swelling agent mixture is the same, no matter what kind of swelling agent is introduced during the oxidative polymerization of DMP.