Biomass gasification is a sustainable way to convert biomass residues into valuable fuels and chemicals via syngas production. However, several gas impurities need to be removed before the final synthesis. Understanding of the interactions and effects of biomass-derived producer gas contaminants (S and K) on the performance of reforming catalysts is of great importance when it comes to process reliability and development. In the present study, the steam reforming activity at 800 degrees C of a sulfure-quilibrated nickel catalyst during controlled exposure to alkali species (similar to 2 ppmv K) and in its absence was investigated using real producer gas from a 5 kW(th) O-2-blown fluidized-bed gasifier. Conversions of CH4, C2H4, and C10H8 were used to evaluate the performance of the Ni/MgAl2O4 catalyst and MgAl2O4 support. A significant and positive effect on the catalyst activity is observed with addition of gas-phase KCl. This is assigned primarily to the observed K-induced reduction in sulfur coverage (theta(S)) on Ni-an effect which is reversible. The catalytic contribution of the K-modified pure MgAl2O4 support was found to be significant in the conversion of naphthalene but not for light hydrocarbons. The product and catalyst analyses provided evidence to elucidate the preferential adsorption site for S and K on the catalyst as well as the role of the support. Whereas S, as expected, was found to preferentially adsorb on the surface of Ni particles, forming S-Ni sites, K was found to preferentially adsorb on the MgAl2O4 support. A low but still significant K adsorption on S-Ni sites, or an effect on only the fraction of exposed Ni surface area near the metal-support interface, can, however, not be excluded. The result suggests that an improved Ni/MgAl2O4 catalyst activity and an essentially carbon-free operation can be achieved in the presence of controlled amount of gas-phase potassium and high sulfur coverages on Ni. Based on the results, a mechanism of the possible K-S interactions is proposed.