A transient lean-rich switching method has been developed in conjunction with the spatially resolved capillary inlet mass spectrometry (SpaciMS) technique using CO oxidation as a probe reaction. This has allowed in-situ spatiotemporal mapping of gaseous species and temperature profiles inside the channels of commercial three way 1%Pd/0.07%Rh/Al2O3/CexZr1-xO2 and 3%Pd/Al2O3 monolith catalysts. The identification of the spatiotemporal reactant conversion, intermediate and product formation, as well as the associated heat evolution, has helped in the elucidation of reaction sequences and furthermore, the postulation of reaction mechanisms, both axially and temporally inside the monolith catalyst. Intriguingly in this work, the SpaciMS technique has provided previously unobserved insights into an apparent excess OSC-like behaviour of the 3%Pd/Al2O3 catalyst, where excess CO conversion is detected under rich CO oxidation conditions in the presence of water. It has been demonstrated that the water gas shift reaction is insufficient to account for the excess CO conversion in this work, therefore a Pd(OH)x surface species is hypothesised to be acting in the same way as an oxygen storage component under rich conditions.