We measured photochemical mineralization of dissolved organic carbon in a humic lake in situ. At a depth of 1 cm, solar radiation mineralized 19 mmol C m Ϫ3 d Ϫ1 . The rate of mineralization decreased with increasing depth with an attenuation coefficient of 23 m Ϫ1 . Consequently, most photochemical mineralization in the water column (0.99 mmol C m Ϫ2 d Ϫ1 ) took place in the top 10 cm. The rate of photochemical mineralization was also modeled as a product of three spectra: (1) scalar photon flux density, (2) the apparent quantum yield ( ), and (3) the absorption of chromophoric dissolved organic matter. We described the spectrum for apparent quantum yield as ϭ c ϫ 10 Ϫ d , where c (dimensionless) and d (nm Ϫ1 ) are positive constants. Mathematical optimization for the best fit between the measured and the modeled photochemical mineralization resulted in of 7.52 ϫ 10 Ϫ0.0122 . The based on the measurements in situ agreed with determined in a laboratory at 320, 355, and 390 nm. Using the determined , we calculated that UV-B contributed 9%, UV-A 68%, and visible light 23% to the photochemical mineralization. Half of total photochemical mineralization was due to wavelengths Ͻ360 nm. Our method for the determination of is applicable in situ, improves the prediction of photochemical reaction rates in surface waters, and offers an alternative to the determination of quantum yields at discrete wavelengths. helsinki.fi). AcknowledgmentsWe thank Anne Ojala for the global radiation measurements, Jorma Keskitalo for many kinds of assistance, Heikki Haario for advice with the Matlab software, and Martti Heikinheimo for advice in the problems of radiative transfer in the atmosphere. The quantum yields at single wavelengths were determined in the laboratory of Helge Lemmetyinen under his supervision.