The value of the yellow substance absorption coefficient in the visible-light spectrum is a very important characteristic used in the optical classification of seawater. This quantity also needs to be included in algorithms for the remote detection of optically active seawater constituents. An extensive database of measurements of optical parameters has been collected during a series of cruises since 1993 in different water masses of the southern Baltic. The yellow substance absorption coefficient at a wavelength of 400 nm was measured and the yellow substance absorption spectrum slope coefficient calculated. The statistical distribution of values of these parameters in the three various water masses (bay waters, coastal, and open seawater) is presented. The seasonal variability of both parameters in the three regions is analyzed. Results are discussed and compared with the published record of yellow substance absorption properties and variability patterns of other optical parameters in the southern Baltic and other marine basins. ¿ 1999 American Geophysical Union