We point out that the simultaneous use of two actinometers can be useful in photochemical quantum yield determinations and illustrate this by describing a simple, general, and mathematically rigorous procedure for the determination of quantum yields of a clean photoreversible photochemical reaction of the type A + hv --> B, B + hv --> A. This is useful when simplifying assumptions cannot be made: both A and B absorb light to a significant extent, irradiation is not at an isosbestic point, and neither the absorbance nor the transmittance of the sample is small. The method represents a viable alternative to those currently in use. It is versatile in that it considers the reaction behavior accurately along its course from an initially pure single component all the way to the photostationary mixture. The performance of the method has been demonstrated on a photoreversible 6pi + 6pi photocycloaddition reaction. A detailed description of an apparatus and procedures suitable for measurement of photochemical quantum yields as a function of wavelength and temperature is provided.