Molecular weight (MW) is a key control on the physical, chemical, and biological characteristics of dissolved organic mater (DOM). This study investigated the effect of photooxidation on the average MW of DOM by exposing DOM of diverse origins to simulated solar radiation at varying levels of dissolved oxygen and under different incident light wavelength regimes. During irradiation, high-molecular-weight fractions were destroyed and low-molecular-weight constituents were formed. The average MW decreased with irradiation time in all treatments in a manner that can be described by a quasi-exponential function, which suggests that solar radiation is incapable of completely mineralizing DOM even after prolonged exposure. Increasing the oxygen concentration accelerated the MW reduction while the removal of oxygen strongly suppressed this process. The fractional contributions from UV-B, UV-A, and visible radiations to full-spectrum photoinduced MW reduction varied considerably among the DOM samples examined, ranging from 19% to 60%, 17% to 36%, and 15% to 46%, respectively. The MW changes in time-series irradiations were inversely correlated with the ratio of the absorbance at 250 nm to that at 365 nm (i.e., the E(2)/E(3) quotient). Photoinduced MW reduction was accompanied by a decrease of polydispersity, which is indicative of a reduced DOM heterogeneity.