The hydrogenation of CO on a Co/TiO2 catalyst under transient conditions was investigated experimentally using the PTK method. By switching between synthesis gas and hydrogen and measuring the transients between the two operating points, the amount of carbon stored on the catalyst surface was evaluated based on material balances. An internal standard enabled the separation between residence time and kinetic effects on the measured transient response. By varying the build-up duration, the amount of carbon accumulated at the surface was affected, which led to the following main conclusions, illustrated in fig. 1. There are at least two carbon species of different reactivity present on the catalyst surface, namely Cα and Cβ.