Solution-mediated doping of polymer optical fibers (POFs) is a facile approach to obtain functional POFs. However, previous studies revealed that POFs may be severely impaired by the doping solution based on methanol or other organic solvents, and the fiber loss could substantially increase due to the solution doping process. This study proposes to dope POFs in aqueous solution dissolved with Rhodamine 6G (Rh6G) at elevated temperature. It is found that absorbed water molecules in POFs lead to fiber expansion at elevated temperature and produce voids large enough for Rh6G to diffuse into the fiber. Experimental results show that Rh6G can be successfully doped into the cladding of POFs in water within the temperature range of 60–80 °C. It is also found that raising temperature can significantly accelerate the doping process and lead to more Rh6G molecules diffusing into POFs. Most importantly, loss of POFs after doping in water even up to 80 °C doesn't increase, in contrast with doping in methanol or other organic solvents. In sum, doping of POFs in water is feasible and even preferable to organic solvents, though it requires that the dopant must be soluble in water. [ABSTRACT FROM AUTHOR]