Spectral absorption coefficients and fluorescence quantum efficiencies were determined for humic substances from a variety of sources. Specific absorption coefficients, K(h), for humic substances at wavelengths lambda from 300 to 500 nm can be closely described by the relation Ae supB(450-lambda), where A and B are constants. When the K(h) values are in units of liter (mg organic carbon) meter and wavelength lambda is in nanometers, mean values of A and B for aquatic humus in the 12 water bodies studied were 0.6 + or - 0.3 and 0.014 + or - 0.001, respectively. Spectral absorption coefficients of dissolved organic matter in blackwater rivers, of the 'yellow substance' in the sea, and of fulvic acids extracted from soils are very similar. Fluorescence quantum yields of humic substances were low and more variable than the absorption coefficients, ranging from 0.0005 to 0.012 with excitation at 350 nm (average of 0.0045 + or - 0.0038 for 6 waters). Fluorescence spectra for the humic substances were remarkably similar with maximum emission at 430 to 470 nm. Results of this study can be used to compute photolysis rates of pollutants as a function of depth in natural water bodies.