Photochemical reactions are initiated by the absorption of ultraviolet or visible light, and they have many important applications in chemical and materials sciences. Topical examples include the use of photoredox catalysis in both organic synthesis and the controlled growth of polymers. The light-induced chemistry often follows a complicated sequence of reaction steps, each involving short-lived reactive intermediates such as free radicals or electronically excited molecules. The chemistry spans multiple timescales, from the ultrafast (femtosecond to picosecond) excited-state dynamics of the molecular chromophores responsible for light absorption to the nano-, micro- or millisecond diffusive reactions of the transient reactive intermediates. Using our in-house ultrafast laser system and the LIFEtime laser facility at the UK’s Rutherford Appleton Laboratory,1-4 we have mapped the multi-step reaction sequences of a range of photochemical reactions in solution by time-resolved infra-red spectroscopy. These measurements identify the transient intermediates through their IR spectral signatures, and quantify the kinetics of each reaction step.5,6 The spectroscopic capabilities allow direct observation of the sequence of steps in complex reaction cycles, and the outcomes provide detailed mechanistic insights. This talk will present the Bristol group’s recent work on the study of two classes of reactions. The first example is photoredox-controlled atom-transfer radical polymerization chemistry using organic photocatalysts based on dihydrophenazine, phenoxazine and phenothiazine core motifs.7 Our experimental studies resolve structure and solvent dependent propensities for electron transfer from either the singlet or triplet excited states of the photocatalysts, and we observe in real time the subsequent radical addition reactions.8 The second example is mechanistic investigation of a photoredox scheme using azide anions as H-atom transfer catalysts. The radical reaction cycles are tracked over several steps from ultrafast initiation by transient absorption spectroscopy.9 The mechanistic understanding that is emerging from this work should contribute to improved designs of photoredox catalysts, and better harnessing of radical reactions in organic synthesis.