Oxygenates (i.e., ethanol and MTBE as required additives in gasoline) and moisture were found to have strong inhibiting effects on desulfurization by adsorption with zeolite. The effects of each individual molecule were studied quantitatively by using a model fuel (500 ppmw thiophene in 80% n-octane + 20% benzene). Cu(I)Y was used as the pi-complexation sorbent. Ab initio molecular orbital calculations showed that the adsorption bond energies with Cu(I)Y were: 21.4 kcal/mol for thiophene; 31.0 kcal/mol for MTBE and 41.6 kcal/mol for ethanol. Separation or selectivity factors can be estimated from heats of adsorption, and the inhibiting effects were predicted to follow the order of the relative heats of adsorption: water > ethanol > MTBE > thiophene. The inhibiting effects were measured by the decreases in the desulfurization capacities of Cu(I)Y in the presence of each additive in the model fuel. The results were in agreement with the theoretical prediction. In addition, the desulfurization capacity was strongly dependent on the liquid hourly space velocity because of the diffusion limitation of thiophene in the zeolite crystals. (C) 2006 Elsevier B.V.. All rights reserved.