Abstract: The invention relates to an apparatus for the continuous measurement of SO3 and/or H2SO4 concentrations in gases using a photometer having a light source, a cuvette, a receiver, an optical filter unit having at least one optical filter which is selected such that it lets a measurement wavelength pass through which is selected such that it is absorbed as much as possible by SO3 and/or H2SO4 and is absorbed as little as possible by the other components of the gas and the photometer is calibrated with SO3 and H2SO4 gases of known concentrations and having an evaluation unit which includes a memory unit, in which already measured transverse sensitivities at the measurement wavelengths are stored and the evaluation unit can determine a concentration value for SO3 and/or H2SO4 continuously from the photo signals and the stored transverse sensitivities.