An experimental protocol for the determination of gas phase absorption cross-sections, and calculation of maximum photolysis rates, has been developed and is described in detail. Utilization of this protocol will provide a basis for evaluating the possible relative importance of one atmospheric reaction pathway (i.e., photolysis) for organic substances which may be emitted into the environment. The experimental technique involves measuring the absorption spectrum over the wavelength region 285-825 nm at various known gas phase concentrations of the test compound in one atmosphere of ultra-pure air. From the measured absorbance (averaged over 10 nm wavelength regions) at the known concentrations of the test compound, absorption cross-sections (again averaged over 10 nm wavelength increments) can be calculated. These absorption cross-sections, together with solar flux data from the literature, then permit calculation of the photolysis rates under atmospheric conditions. Since a photolysis quantum yield of unit is assumed in these calculations, the resulting photolysis rates are upper limits. The relative importance of this calculated maximum photolysis rate as an atmospheric reaction pathway, relative to reaction with ozone and with the hydroxyl radical, can then be assessed. If the photolysis rate is shown to be of importance, then further experimental data on the quantum yield for photolysis under atmospheric conditions is required to precisely determine the actual photolysis rate.