Hydrogen is currently mainly produced via steam reforming of methane (SMR: CH 4 + H 2 O → CO + 3H 2 ). An alternative to this process, utilizing carbon dioxide and thus potentially mitigating its environmentally harmful emissions, is dry methane reforming (DMR: CH 4 + CO 2 → 2CO + 2H 2 ). Both of these reactions are structure sensitive, that is, not all atoms in a catalytic metal nanoparticle have the same activity. Mapping this structure sensitivity and understanding its mechanistic workings provides ways to design better, more efficient, and more stable catalysts. Here, we study a range of SiO 2 -supported Ni nanoparticles with varying particle sizes (1.2−6.0 nm) by operando infrared spectroscopy to determine the active mechanism over Ni (carbide mechanism) and its kinetic dependence on Ni particle size. We establish that Ni particle sizes below 2.5 nm lead to a different structure sensitivity than is expected from and implied in literature. Because of the identification of CH x D x species with isotopically labeled experiments, we show that CH 4 activation is not the only rate-limiting step in SMR and DMR. The recombination of C and O or the activation of CO is likely also an important kinetically limiting factor in the production of synthesis gas in DMR, whereas for SMR the desorption of the formed CO becomes more kinetically limiting. Furthermore, we establish the Ni particle size dependence of carbon whisker formation. The optimal Ni particle size both in terms of activity for SMR and DMR, at 500 and 600 °C, and 5 bar, was found to be approximately 2−3 nm, whereas carbon whisker formation was found to maximally occur at approximately 4.5 nm for SMR and for DMR increased with increasing particle size. These results have direct practical applications for tuning of activity and selectivity of these reactions, while providing fundamental understanding of their working.