More than 1500 water samples were taken from the Kattegat, the Skagerrak and adjacent waters. The value of the absorption coefficient of yellow substance at 310 nm was found to vary from 0.06 to 7.4 m^-1 in the open coastal waters, with a mean value of 1.3 m^-1. The corresponding wavelength-averaged value (250-450 nm) of the semilogarithmic spectral slope of the coefficient ranges from 0.008 to 0.042 nm^-1, and the mean value is 0.023 nm^-1. Closer to river discharges, as in the fjords, the values of the slope seem to be more constant at around 0.0175 ± 0.0015 nm^-1. In this area the slope must then be known in order to compare absorption at different wavelengths or to model the yellow substance absorption.