Isotopic transient tracer techniques have been employed to study heterogeneous hydroformylation on Rh/SiO2 and Mn-Rh/SiO2. Pulse injection of D-2 and (CO)-C-13 allowed tracing of the deuterium and CO incorporation pathway into the aldehyde product. The d(1)- and d(2)-propionaldehyde responses showed a double-peak, or two-hump, response to the D-2 pulse, while C2H5 (CHO)-C-13 showed a single-hump response to the (CO)-C-13 pulse. Analysis of the product responses to the D-2 pulse in CO/H-2/C2H4 and CO/H-2/C2H4/C2H5CHO suggests that the first hump of the d(1)- and d(2)-propionaldehyde responses was due to rapid H/D exchange with adsorbed propionaldehyde via enol intermediates. The decay of the second hump was due to reaction of adsorbed acyl with spillover hydrogen/deuterium. The C2H5 (CHO)-C-13 response was due to CO insertion followed by acyl hydrogenation. Compartment modeling of the product responses from the (CO)-C-13 and D-2 pulse inputs allowed determination of residence times of adsorbed intermediates, surface coverages of adsorbed intermediates, and the elementary rate constants for acyl hydrogenation and CO insertion. Elementary rate constants for acyl hydrogenation determined from this study were consistent with the value calculated by transition state theory (TST). The addition of Mn promoter to Rh/SiO2 increased coverages of *CO, *C2H5, and *C2H5CO and shifted the rate-limiting step for propionaldehyde formation. Acyl hydrogenation is the rate-limiting step on Rh/SiO2 while CO insertion and acyl hydrogenation are both kinetically significant on Mn-Rh/SiO2.