The invention relates to a method for monitoring the condition of a medium in a channel, based on the transmission/emission of light, in which a light at a set wavelength is conducted through a medium layer defined by a measuring gap in a measuring head pushed in from an opening in the wall of the channel the intensity of the light passed through medium layer, or a variable proportional to it is measured, and the condition of the medium is evaluated, using measuring electronics, from the change of the intensity, according to established criteria. In the method, the wavelength of light used is such that the resolution of the aging phenomenon of the medium being monitored is optimal and the relationship of the temperature dependence of the medium to the measuring variable is taken into account. In addition, the invention also relates to a corresponding device.