This paper provides an overview of the use of various transient infrared methods to determine the role of infrared observable species in the mechanisms of the NO–CO reaction, heterogeneous ethylene hydroformylation, and photocatalytic oxidation of ethanol. The transient infrared methods with a judicious choice of ways in changing the concentration of reactants and their isotope counterparts produce responses, allowing (i) identification of the spectators, (ii) determination of active adsorbed species, and (iii) verification of kinetic models and their parameters. The method has also been recently extended to monitor infrared absorbance of photogenerated electrons during photocatalysis, correlating variation in the concentration of photogenerated electrons and adsorbed species. The specific discussion focuses on limitations of the approaches and the type of mechanistic information that can be obtained.