Deuterium isotopic pulse tracing has been combined with in situ infrared spectroscopy to study heterogeneous ethylene hydroformylation and hydrogenation on a 4 wt% Mn-Rh/SiO2 catalyst at 513 K and 0.1 MPa. Deuterium pulses into steady-state CO/H-2/C2H4 how produced H-2, HD, D-2, C2H6-xDx, and C2H6-xDxCO transient responses. The reaction parameters, including intermediate surface coverages and rate coefficients, were determined from the transient responses coupled with compartment models that describe adsorbed intermediates in the pathway for incorporation of D-2 into propionaldehyde. This pathway is shown below: GRAPHICS The pathway involves (i) deuterium adsorption, (ii) partial deuteration of adsorbed ethylene to form adsorbed ethyl, (iii) deuteration of adsorbed ethyl to form ethane, (iv) CO insertion into adsorbed ethyl to form adsorbed acyl, (v) deuteration of adsorbed acyl via metal-chemisorbed deuterium to form propionaldehyde, and (vi) deuteration of adsorbed acyl via spillover deuterium to form propionaldehyde. The steady-state rate of hydrogen desorption, which is essential for determination of the reaction parameters, was estimated by use of the HD transient response assuming that the rate of HD production equals the rate of D-2 production and equals the rate of H-2 production at the point where the surface coverages of H and D are approximately equal. Evaluation of rate coefficients indicates that hydrogenation of adsorbed ethyl is intrinsically faster than CO insertion into ethyl, leading to the high selectivity toward ethane over propionaldehyde as product. The propionaldehyde compartment models were able to account for two modes of acyl hydrogenation: (i) from *H on the metal surface and (ii) from spillover *H, or Si-OH. Hydrogenation via metal-adsorbed hydrogen is favored over hydrogenation via spillover hydrogen. Comparison of the rate coefficients for CO insertion and acyl hydrogenation indicates that both steps are kinetically significant, which is consistent with conclusions of previous studies that utilized the Langmuir-Hinshelwood-Hougen-Watson and pseudo-steady-state analysis approaches.