Thermodynamic properties of glycerin steam reforming have been studied with the method of Gibbs free energy minimization for hydrogen and/or synthesis gas production. Equilibrium compositions including the coke-formed and coke-free regions were determined as a function of water/glycerin molar ratios (1:1-12:1) and reforming temperatures (550-1200 K) at different pressures (1-50 atm). Optimum conditions for hydrogen production are temperatures between 925 and 975 K and water/glycerin ratios of 9-12 at atmospheric pressure, whereas temperatures above 1035 K and water/glycerin ratios between 2 and 3 at 20-50 atm are suitable for the production of synthesis gas that favors both methanol synthesis and low-temperature Fischer-Tropsch synthesis. However, synthesis gas obtained from glycerin steam reforming is not feasible for direct use in high-temperature Fischer-Tropsch synthesis. Under these optimum conditions, carbon formation can be thermodynamically inhibited.