Computational catalytic studies indicate that the elementary reactions that constitute the Fischer–Tropsch reaction strongly dependent on the structure of the catalyst reaction center. Recent experimental evidence is available that, for metallic Fischer–Tropsch catalysts such as Co or Ru, the very small metallic particles show altered catalytic performance. To distinguish between changes in the relative concentration of reaction centres, changes in chemical reactivity, or rate controlling steps, transient SSITKA data are extremely useful. Here, we present kinetics simulations to extract molecular kinetic information from SSITKA data. We have applied such simulations to interpret published experimental SSITKA data on nano-particle size dependent Fischer–Tropsch (FT) kinetics. The FT catalytic cycle consists of four essential reaction steps. Their relative size determines activity as well as selectivity. The simulated SSITKA indicate three different regimes with different kinetic behaviour, where the two fundamental regimes to distinguish are the monomer-formation-limited and the chain-growth-limited regime. Particle size changes shift kinetics from one to the other regime. We note different effects of supports and choice of metal composition on changes in elementary rates or the relative number of reactive centres when the particle size is decreased in the nanometre regime.