Deuterium isotopic step tracing combined with in situ infrared (IR) spectroscopy was utilized to study and model the hydrogenation steps in ethylene hydroformylation on 4 wt% Rh/SiO2 at 483-573 K and 0.1 MPa. The difference in residence times between H-2 and D-2, as well as propionaldehyde and deuterated propionaldehyde to the step switch from H-2 to D-2 and D-2 to H-2 during ethylene hydroformylation reflects the presence of an isotope effect for H-2/D-2 chemisorption and propionaldehyde formation. Compartment modeling of H-2/D-2 responses and qualitative comparison of propionaldehyde and deuterated propionaldehyde responses unravel the presence of a normal equilibrium isotope effect for H-2/D-2 chemisorption and a normal kinetic isotope effect for hydrogenation/deuteration of adsorbed acyl species. In situ IR coupled with deuterium transient responses shows that the reverse spillover of deuterium from Si-OD participates in the deuteration of adsorbed acyl species, suggesting that the site for deuteration of the adsorbed acyl species is located near the Rh and SiO2 interface. Significant difference in the deuterated ethane and propionaldehyde responses suggests the different nature of either adsorbed hydrogen/deuterium or the active site for hydrogenation/deuteration of adsorbed ethyl and acyl species.