Co3O4nanoparticles were supported on different TiO2polymorphs, namely, rutile, anatase, and a 15 : 85 mixture of rutile and anatase (also known as P25), viaincipient wetness impregnation. The Co3O4/TiO2catalysts were evaluated in the preferential oxidation of CO (CO-PrOx) in a H2-rich gas environment and characterised in situusing PXRD and magnetometry. Our results show that supporting Co3O4on P25 resulted in better catalytic performance, that is, a higher maximum CO conversion to CO2of 72.7% at 200 °C was achieved than on rutile (60.7%) and anatase (51.5%). However, the degree of reduction (DoR) of Co3O4to Co0was highest on P25 (91.9% at 450 °C), with no CoTiO3detected in the spent catalyst. The DoR of Co3O4was lowest on anatase (76.4%), with the presence of TixOy-encapsulated CoOxnanoparticles and bulk CoTiO3(13.8%) also confirmed in the spent catalyst. Relatively low amounts of CoTiO3(8.9%) were detected in the spent rutile-supported catalyst, while a higher DoR (85.9%) was reached under reaction conditions. The Co0nanoparticles formed on P25 and rutile existed in the fcc and hcp crystal phases, while only fcc Co0was detected on anatase. Furthermore, undesired CH4formation took place over the Co0present in the P25- and rutile-supported catalysts, while CH4was not formed over the Co0on anatase possibly due to encapsulation by TixOyspecies. For the first time, this study revealed the influence of different TiO2polymorphs (used as catalyst supports) on the chemical and crystal phase transformations of Co3O4, which in turn affect its activity and selectivity during CO-PrOx.