Under UV irradiation, an important primary photochemical reaction of colored dissolved organic matter (CDOM) is electron ejection to produce hydrated electrons (e-aq). The efficiency of this process has been studied in both fresh water and seawater samples with both steady-state scavenger (S-SS) and time-resolved laser flash photolysis (LFP) methods. However, the apparent quantum yields (AQYs) of e-aq for the same samples using the two methods differ by as much as a factor of 100, necessitating a closer re-examination of how the process is measured. We developed a highly sensitive multipass LFP apparatus that allows detection of transient species at very low and variable UV irradiation intensities. Under single-photon conditions, we measured the AQY of e-aq from Laurentian fulvic acid as 1.3 x 10(-4), and set the upper limit for other CDOM samples at 6 x 10(-5), bringing the LFP results into agreement with those from S-SS methods. We also examined the ionization at elevated irradiation intensities and clearly demonstrated that multiphoton ionization occurs at intensities well below those usually used in LFP experiments, but well above those likely to occur at the earth's surface. This multiphoton ionization is probably the cause of the high AQYs reported by earlier LFP work. In addition, we also observed in real time other photochemical reactions, such as triplet quenching and bleaching, in the single photon regime.