Singlet oxygen ((1)O2) plays an important role in the inactivation of pathogens and the degradation of organic contaminants. The present study looks at the surface steady-state concentration of (1)O2 and quantum yields (ΦSO) for organic matter present in or derived from wastewater (WWOM), including those that are partially treated and after undergoing oxidation. The surface steady state concentrations of (1)O2 ranged from 1.23 to 1.43 × 10(-13) M for bulk wastewaters under simulated sunlight. The ΦSO values for these samples varied from 2.8% to 4.7% which was higher than the values observed for the natural organic matter isolates evaluated (1.6-2.1%). Size fractionation of WWOM resulted in ΦSO increases, with a value of up to 8.6% for one of the <1 kDa fractions. Furthermore, oxidation of WWOM by hypochlorous acid (HOCl) and molecular ozone also resulted in an increase in ΦSO, with the highest measured value being 9.3%. This research further explores the correlations between the photosensitizing properties of WWOM and optical characteristics (e.g., absorbance, E2:E3 ratio). Making use of easily measurable absorbance values, a model for the prediction of (1)O2 steady-state concentrations is proposed.