Quantifying singlet oxygen production from triplet photosensitizers typically requires spectroscopic techniques and specialty chemical traps. Herein we describe a novel method for determining the singlet oxygen quantum yield (фΔ) of photosensitizers. The method utilizes the reaction between singlet oxygen and dimethyl sulfoxide (DMSO) to produce dimethyl sulfone. The rate of the reaction is measured by the pressure decrease that results from the consumption of oxygen from the headspace of a sealed system. It was found that the rate of pressure decrease was directly related to the singlet oxygen quantum yield. The фΔ from known photosensitizers: fluorescein, Eosin Y, Eosin B, methylene blue, and tris(bypridine)ruthenium (II) in DMSO were determined by the rate of pressure decrease using Rose Bengal as a relative standard. The singlet oxygen quantum yields of the photosensitizers in DMSO were found to be in agreement with the reported values for each photosensitizer determined using traditional spectroscopic methods.