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^-3 d^-1. The rate of mineralization decreased with increasing depth with an attenuation coefficient of 23 m^-1. Consequently, most photochemical mineralization in the water column (0.99 mmol C m^-2 d^-1) 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 _ = c x 10^-dl, where c (dimensionless) and d (nm^-1) are positive constants. Mathematical optimization for the best fit between the measured and the modeled photochemical mineralization resulted in _ of 7.52 x 10^-0.0122l . 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 <360 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.