a method for oxidizing a substrate which is oxidizable by singlet oxygen is discolded . the first step of the method comprises establishing a flow to and from each of a plurality of successive closed zones , each of which zones contains a quantity of a polymer - bound photosensitizing catalyst that is a source for singlet oxygen when irradiated by light of a particular wavelength , and is of such size and shape that a major portion of the catalyst therein can be made a source for singlet oxygen by irradiation with light of a suitable wavelength . the flow is established by charging the substrate in the liquid phase into the first zone , removing reaction product from the last zone , and transferring reaction product from the first zone and from each intermediate zone to the next succeeding zone . oxygen is introduced into each of the zones and each is irradiated with light of a frequency which causes the catalyst to be a source for singlet oxygen .