Summary The results of this work show that the time-resolved 1270-nm IO2 luminescence can be used as an effective probe for the mechanism of photooxygenation reactions. If@ lO2 is affected by substrate, it is necessary to look for substratesensitizer interactions and probe the system further. If only the IO2 lifetime is changed by substrate, a pure IO2 Type I1 mechanism is involved. This work also suggests that the production of IO2 from sensitizers ...