Photochemical degradation is an important issue to be overcome in advancing the lifetime of fluorene-containing conjugated polymers. In order to optimize the inertness of the materials, a quantitative measure for the efficiency of degradation is needed. Here, we introduce a method to measure a relative quantum yield of the photochemical degradation by monitoring the kinetics of the process by means of UV/vis spectroscopy and liquid chromatography (LC) techniques. This method is employed to a set of differently substituted 2,7-diphenylfluorenes, serving as model compounds for polyfluorene materials. Our measurements show that the quantum yield changes by orders of magnitude upon varying the bridge substituents and that altered kinetics indicate changing degradation mechanisms.