We demonstrate a transient rate model for photochemical upconversion that links the internal energy transfer and triplet–triplet annihilation processes to spectroscopically measurable quantities, such as delayed fluorescence and bleaching. We confirm that our model is able to reproduce published delayed fluorescence measurements extremely well. We then use transient absorption spectroscopy to directly observe the dynamics of triplet populations through clear observation of delayed bleaching of the emitter species, providing direct evidence of triplet energy transfer from sensitiser to emitter molecules. This more complex experiment is also well reproduced by our model.