The kinetics and diffusion effects of methanol steam reforming reaction over commercial Cu/ZnO/Al2O3 catalyst was studied for steam to methanol (S/M) ratios of 0 to 1 and pressures below 6 bar. Our objective is the development of a novel high-pressure propulsion technology based on the concepts of thermochemical recuperation (TCR) and onboard hydrogen production. A simple kinetic model assuming methanol decomposition followed by water-gas-shift was used to estimate the rate constants (k(MD), k(WGS)). The apparent activation energy of k(MD) was estimated as 45-55 kJ/mol for large pellets and S/M = 1.0, 0.5, and 0.0; k(MD) for S/M = 0 (and the conversions obtained) were smaller than those of S/M = 1, probably due to CO inhibition. At temperatures above similar to 500 K, the WGS is at equilibrium. Strong pore-diffusion limitations are evident at 1 bar for the 3 mm catalyst, evident experimentally as well as by analysis; the apparent k(MD) is almost diffusion free for particles of 0.7 mm in diameter. This resistance increases, of course, with P. The selectivity of CO (dry basis) increases with W/Fmeth (weight of the catalyst divided by flow rate of methanol in units of kgcat s mmol(-1)) and increases with decreasing particle size. As the pressure increases, the ratio of CO and CO2 increases moderately up to 4 bar; at 6 bar the change is drastic. Similar observations were made with S/M = 0.5 and 1.0. Deactivation rates and coke formation were also studied and were found to be marginal under atmospheric pressures over a period of 10 h and became evident only at S/M = 0 and 275 degrees C; at 6 bar the decline was evident already at S/M = 0.5. The source of deactivation was attributed to coking, a conclusion based on TPO of spent catalyst. In the case of S/M = 1 and 0.5, and at 6 bar, there was a shift in the composition from CO2 to CO with time over a period of 10 h at 275 degrees C. However, the carbon deposition in all cases was estimated to be about the same.