Abstract Chemical actinometry can be used to determine photons absorbed for a photochemical reaction, which is required to calculate the quantum yield. A photochemical reaction with a known quantum yield can be used as a relative standard for the determination of an unknown quantum yield for a light-driven reaction. Herein, we have developed a simplified approach to using the popular potassium ferrioxalate actinometer. Traditionally, the photoreduction of Fe(III) to Fe(II) is monitored by following the absorbance of Fe(II) by reacting aliquots of the actinometry solution with 9,10-phenanthroline to form a red colored complex. The multiple steps for this method make it tedious and vulnerable to errors, especially inadvertent light exposure. In lieu of spectroscopic measurements of the Fe(II) concentration, the production of CO2 was measured to determine the number of photons absorbed over time. CO2 production was measured in two different ways: by the pressure increase in a sealed system and the volume change by trapping the CO2. Both methods were considerably less laborious and showed agreeable results compared with the traditional spectroscopic method.