A method for cleaning and conditioning the water-steam circuit of a power plant, especially of a nuclear power plant, uses an amine as a film-forming agent. The amine is metered into the working medium circulating in the water-steam circuit. The film-forming agent forms a hydrophobic film on the surfaces of the circuit. During the process, the concentration of at least one impurity contained in the water and the concentration of the film-forming agent in at least in the feed water of the steam generator are measured and monitored. The concentration of the film-forming agent is varied, as needed, subject to the concentration of the impurity.