The curve of photodestruction quantum yield versus photon energy for tryptophan and phycocyanin has an inverted region where the quantum yield decreases as the absorbed photon energy increases. A formula is proposed to predict the existence of inverted regions in photochemical reactions. This formula is used to describe the inverted behavior of the quantum yield of photodissociation for tryptophan and phycocyanin. The model also describes the quantum yields for the photodissociation of H2O and O-3.