Here we investigate isotope effects on the catalytic methanol synthesis reaction and the reactivity of copper-bound formate species in CO2-H-2 atmospheres on Cu/SiO2 catalysts by simultaneous IR and MS measurements, both steady-state and transient. Studies of isotopic variants (H/D, C-12/C-13) reveal that bidentate formate dominates the copper surface at steady state. The steady-state formate coverages of HCOO (in 6 bar 3:1 H-2:CO2) and DCOO (in D-2:CO2) are similar and the steady-state formate coverages in both systems decrease by similar to 80% from 350 K to 550 K. Over the temperature range 413 K-553 K, the steady-state methanol synthesis rate shows a weak H/D isotope effect (1.05 +/- 0.05) with somewhat higher activation energies in H-2:CO2 (79 kJ/mole) than D-2:CO2 (71 kJ/mole) over the range 473 K-553 K. The reverse water gas shift (RWGS) rates are higher than methanol synthesis and also shows a weak positive H/D isotope effect with higher activation energy for H-2/CO2 than D-2/CO2 (108 vs. and 102 kJ/mole) The reactivity of the resulting formate species in 6 bar H-2, 6 bar D-2 and 6 bar Ar is strongly dominated by decomposition back to CO2 and H-2. H-2 and D-2 exposure compared to Ar do not enhance the formate decomposition rate. The decomposition profiles on the supported catalyst deviate from first order decay, indicating distributed surface reactivity. The average decomposition rates are similar to values previously reported on single crystals. The average activation energies for formate decomposition are 90 +/- 17 kJ/mole for HCOO and 119 +/- 11 kJ/mole for DCOO. By contrast to the catalytic reaction rates, the formate decomposition rate shows a strong H/D kinetic isotope effect (H/D similar to 8 at 413 K), similar to previously observed values on Cu(110).