Two models of the mechanism of photoinduced electron transfer at semiconductor surfaces have long been differentiated by their prediction, or their denial, of the existence of a light intensity threshold for fuel-forming photoconversion processes. We attempt to clarify this problem by making a distinction between two possible types of thresholds : a threshold for incipient product formation and a threshold for product formation in a specified state, such as its standard state. A light intensity threshold for incipient product formation appears to be forbidden by molecular electron-transfer theory and has apparently never been observed. Conversely, a light intensity threshold for product formation in its standard state must always occur, simply because the product concentration must first build up from its equilibrium value to its standard-state value. Since the former threshold is forbidden, while the latter is unavoidable, the existence of a threshold cannot be used to distinguish between the models.