Combined FT-IR/mass spectrometry was used to study the dynamic behavior of hydrogen/deuterium adsorbates in response to D-2 and C2D4 pulses during ethylene hydroformylation on Mn-Rh/SiO2 at 513 IC. and 0.1 MPa. Variation of exposure time of the catalyst to the D-2 and C2D4 pulses revealed that readsorption did not play a significant role in the product responses; long exposure time allows more deuterium to enter the slower pathway of d(1)- and d(2)-propionaldehyde, The two-unequal-hump HD response from both the D-2 and C2D4 pulses indicates the presence of an isotope effect on hydrogen/deuterium adsorption. The rapid responses of the deuterated ethylene and ethane can be described by the Horiuti-Polanyi mechanism and indicate that their intermediates have small theta (surface coverage) and tau (residence time) which cannot be resolved by the time resolution of their transient responses, The d(1)- and d(2)-propionaldehyde products show a two-hump response, which can be explained by the different deuterium incorporation pathways into the propionaldehyde. The first hump is due to hydrogen/deuterium exchange on adsorbed acyl and acyl hydrogenation, The second hump is due to H/D exchange on the adsorbed alkyl species, which then undergoes CO insertion and acyl hydrogenation, The second hump decay is partly due to the reaction with Si-OD as indicated by the parallel decay of d(1)- and d(2)-propionaldehyde and Si-OD.