Since the early 1980s, the geostationary Meteosat instruments have been observing the earth at a high temporal and spatial resolution. The data record contains more than 30 years of observations and has become very useful for climate monitoring. With a compilation of 10 satellites, it is crucial to reach the highest possible consistency between the instruments. However, the quality of the level 1 data records is still far from what is needed to generate homogeneous climate data records, due to both temporal ageing as well as discontinuities between successive instruments. In this letter, a method is proposed to assess the pre-launch Meteosat-7 visible spectral response curve and a model of its ageing. The results not onldy validate the ageing model, but also provide evidence that the official pre-launch spectral response curve of Meteosat-7 poorly represents the sensitivity of the instrument in the blue part of the visible spectrum. Until the Meteosat-7 visible spectral response curve has been reassessed, it is suggested to replace the Meteosat-7 curve with the curve of the high resolution visible channel of Meteosat-8. Quantitatively, the error introduced when unfiltering the data (i.e. conversion from filtered to unfiltered radiance), due to the uncertainty on the Meteosat-7 spectral response, is estimated at 4.5% using the Meteosat-7 spectral response curve, but could be reduced to 2.1% by following the recommendations in this letter.