The invention relates to a method for doping a liquid medium with a dopant which is formed in situ out of at least two reaction constituents, and relates to a device provided for carrying out said method. According to the invention, the formation of the dopant and the doping are carried out in parallel by feeding the reaction constituents into a pipe-like or tubular second reactor (3) arranged inside a first reactor (2), and the dopant formed while flowing through the second reactor is dosed into the second reactor. The method and the device are especially suited for producing and simultaneously using dopants containing performic acid.