Abstract: A method of determining the weight per unit of area of conversion layers, particularly of phosphate layers, on metals, in which the heat radiated from the dry surface which is covered with the conversion layer and is at the temperature T and the heat radiated from a reference surface or a reference cavity which is at the temperature T+t are periodically measured at a temperature between 15.degree. and 50.degree. C. by a wavelength-insensitive electic infrared detector. t is preferably selected in the range from +15.degree. to +3.degree. C. from -15.degree. to -3.degree. C. above or below T. The weight of the conversion layer per unit of area is determined from the amplitude of the resulting electric alternating signal. The radiant heat is suitable measured by an infrared detector having a sensor which consists of a pyroelectric sheet of polyvinylidene fluoride.